- Sort by Z before duplicating a selection (so that relative Z's are
kept)
- in UBZLayerController, iterate only through UBGraphicsScene's fast
access items, not QGraphicsScene::items

- Defined constants in UBSettings for highlighter preview circle colors
- Switch between light and dark colors when changing backgrounds
- For eraser: use color settings that were defined in UBSettings (they
were ignored since the preview circle was added)

When a video is first loaded (placed on the scene), we play/pause it to
load the first frame; but this was also called when the video was
manually stopped. To avoid this, a mStopped attribute was added to
UBGraphicsMediaItem. It is set to true only when the video is stopped by
the user.

(Solution from previous commit worked only on Linux)

+ clean-up in a few places

That function requires access to variables that are initialized
in videoItem and audioItem constructors, so it can sometimes fail
when called from the superclass's constructor.
It might be an idea to avoid those calls altogether though

- Hovering over the video now makes the seek bar visible
- The size of the video item is no longer changed when the video
finishes playing
- Media errors are now handled by the mediaItem and displayed for the
user

- Code clean-up

- Removed inheritance of UBGraphicsProxyWidget; cleaned up related code
- Added two children classes: UBGraphicsVideoItem and
UBGraphicsAudioItem. UBGraphicsMediaItem is now an abstract class.
- Better encapsulation; the Scene and other external classes no longer
access the mediaObject directly

There is now less distinction between audio and video items to outside
code: apart from the UBSvgSubsetAdaptor, there is no need to know
whether a media item holds a video or audio file. Creation is handled
through the static function `UBGraphicsMediaItem::createMediaItem(URL,
parent)`

- zValue is correctly initialized when duplicating an object
- Duplicated objects are translated correctly

- Added a graphical display of stroke z level
- Enabled BSP tree use in the graphicsScene

- 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.

- 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.

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

Previously, duplication did not copy the transformation matrices of group
members correctly. This made grouped objects move away when saving and
re-opening a document.
This should now be fixed.