Also quick code clean-up in UBDocumentController

When both a folder and items it contained were selected, and trashed,
the documents could sometimes be deleted twice and thus, permanently
deleted when one just intended to move them to the trash.

- When clicking a stroke, they aren't moved immediately anymore; a
certain drag distance is necessary, which makes it easy (again) to
select a stroke with a stylus (which tends to move a little as it is
clicked, hence the problem).

- Removed duplicate code; the movement is now managed by
QGraphicsItemGroup::mouseMoveEvent. This prevents use of the transform()
method to get the stroke's transformation matrix; so sceneTransform() is
used instead when copying a strokes group.

- Also fixed an oversight in UBBoardView: Media items couldn't be moved
directly anymore.

The size of the various parts of the compass can now be easily modified

This makes it possible e.g to use the colorPicker app in conjunction
with the compass.
Colors will still be updated when one clicks the pen or marker tools.

- Modified UBvgSubsetAdaptor to correctly save and load strokes, so the
transform matrices that were saved are now loaded correctly.

- Added handling of mousePress / Move / Release events to
UBGraphicsStrokesGroup, so that the transform matrix is calculated and
stored after moving a pen stroke directly (by clicking on it, and not on
its frame). Note: this duplicates quite a bit of code that is in
UBGraphicsDelegateFrame. It may be best to go back and modify both
classes so that the same functions can be called when moving a stroke.

also corrected the "useSystemOnScreenKeybard" to
"useSystemOnScreenKeyboard".

Mostly, removal of old code that had been commented out instead of
removed

- Moved all platform-specific code to UBPlatformUtils
- For now, removed support (on Linux) for checking whether onboard is 
running when OpenBoard is started (it wasn't working anyway, but it needs a
cleaner solution for all 3 platforms)