This reverts commit 8fec7b23.

fixed a bug where erasing a pen stroke with simplifyPenStrokes=true ereased every overlapping polygons of it

(finished) fixed some bugs where 'Locked' and 'Visible on extended screen' properties were not persisted for groups

(WIP) fixed some bugs where 'Locked' and 'Visible on extended screen' properties were not persisted for groups'

This ensures distance from previous point is calculated correctly

Erasing part of (but not the entire) center cross of an arc drawn with
the compass caused a crash.

-> also deleted obsolete UBInterpolator classes

Distance between the last drawn point and the current point is
calculated to be able to discard very short stroke segments (i.e we only
add to the current stroke if the input device has moved more than a
certain distance since the last drawn point).

This commit moves this code from the stroke to the scene, which allows
to calculate distance more accurately: it is now calculated as the
total, absolute distance traveled since the last point, rather than simply the
length of a line between the last point and current one.

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.

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

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.

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.

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.

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)

- Selecting multiple media items then grouping them didn't behave as it
should for other items => fixed by adding type tests
- A group containing several media items wasn't saved to SVG with those media
items as children, due to incorrect UUID copying in the mediaItems's
deepCopy() methods