Revert "workaround to adress badly attached thumbnail views (no more time to investigate on it...) + prevent overwriting scenes"

This reverts commit 0fd15f62.

workaround to adress badly attached thumbnail views (no more time to investigate on it...) + prevent overwriting scenes

This reverts commit b6f41165.

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.

Removed 'Set as background' button for images, since that feature is now accessible from the image menu on the board

Web results still show the details which include some metadata about the
image retrieved, for example.

This is now a setting so it can easily be re-enabled if desired (which
could be useful especially if we decide to also display metadata of
local items)

-> 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.

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.