This mainly changes document mode behaviour in two ways:
1) When deleting 2+ items, a new document was selected in the list. Now, the current document is selected,
or if it has been deleted, a the first document in the list is set as
current document.

2) When deleting the last item in the trash, no document was selected.
Now, the current document is selected instead.

This adds an option to the menu of an image's frame, to set the image as
background. Undo and redo functions were also implemented.

This also removes the display of the "properties" page in the library
pane when clicking on an application, interactivity or flash animation.
For now, it was kept for other items such as images and videos.

Credit: ciampix

Issue observed was that OpenBoard would crash on some Windows systems
when a video was on the page and that page was saved (due to switching
to document mode, auto saving, or duplicating the page), or when cutting
the video with Ctrl-X.

This was due to QTBUG-32522, where setting the video output for a
QMediaPlayer that is hidden results in a crash.

This commit is a work-around for this Qt issue, and so should be reverted
if and when the upstream issue is fixed.

This fixes two related issues:
1) When taking a partial screenshot of the desktop, then copying it
(Ctrl-C) and pasting it in a new document, it was not saved
2) When a page of one document was copied into another (in document
mode, by dragging the page onto another document), media files
disappeared from the new page.

In some cases, export of a document containing a PDF background to PDF
caused the contents to be truncated.

The "simple" PDF exporter will now set the output page size to be equal
either to the document nominal size or, if the document has a background
PDF item, to the size of this item.

Bug likely surfaced when fixing positioning of saved strokes. Quick
tests show that positioning now works both for duplication and
saving/loading

When deleting a document that was in the trash, the following item was
selected but also set as current document. This is now fixed.

This code was lifted from Open-Sankoré 2.10.

This fixes an issue where erasing part of a stroke that had been moved
or rotated, then clicking "undo" then "redo" would place part of the
stroke in the wrong place.

We can (again) check for updates and, if an update is available, send
the user to the site to download them.

The old format (a .json specifying a version number and download URL)
was kept. The address for this file is now specified in the settings.

Images (UBGraphicsPixmapitem and UBGraphicsSvgItem) can be set as
background via the menu on their frame. Currently, the image is first
centered and un-transformed before being set as background.

The option in the frame appears for any UBGraphicsItem for which
`data(UBGraphicsItemData::ItemCanBeSetAsBackground)` is true.

This is (currently) only enabled for image items.

We can (again) check for updates and, if an update is available, send
the user to the site to download them.

The old format (a .json specifying a version number and download URL)
was kept. The address for this file is now specified in the settings.

Fixes an issue where locked items could be moved if they were selected
along with other items, and these items all moved by dragging the
selection frame.

This implementation prevents any movement of the selected items if at
least one of them is locked. It also changes the colour of the selection
frame, like a locked UBGraphicsDelegateFrame.

When drag-n-dropping a document to the trash, it would reappear in the
"untitled documents" folder on the next restart. (This was introduced by
bd3d8e95)

Added checks for the size of the interior, cut-out triangle to make sure
everything is drawn correctly at small sizes; buttons are now also
hidden if they overflow from the tool.

Also install the importer executable to OpenBoard root install folder

This removes a few instances of deleting a scene twice, or accessing
elements of a scene after they've been deleted.

Previously, the application would crash upon exiting if the scene was
empty but had been modified (e.g if an object was placed on the board
then deleted, then the application closed)

Previously, only transforms were saved -- not positions (which are set
if a group is moved by dragging it directly; if dragged by its frame,
its transform is updated instead).

Issue observed was that a group that had been moved would lose its new
position when the document was saved then loaded. (All other transforms
were kept, however).

Now, when duplicating a group before saving a document, position is
included in the group's transform.

This was observed in some cases on low-resolution screens, at least on
Linux and Windows.

The previously hardcoded value for the width of the text items' titlebar
(consisting of the buttons for formatting text) was replaced by a
method calculating its width (which varies based on screen resolution).

... to the nearest 2 decimal places. This fixes a bug where upon loading
a text item, it could be scaled by e.g 0.999999, which would eventually
round down the point size by 1pt. Making the text item shrink by 1pt
every time the document was opened.

Fixed issue that appeared with previous commit, where grouped strokes'
positions were sometimes saved and loaded incorrectly.

Strokes and their transforms should now be saved correctly whether they
are grouped or not

(Much simpler and cleaner solution than previous commit)

This reverts commit d7b73a16.