- 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.

Application compiles and runs, with some caveats. Full list of changes:

- minor changes related to Qt4->Qt5 API differences

- Replaced calls to Carbon framework by Cocoa

- Removed registering of AE event handler. Seems to be done
automatically in Qt5.

- temporarily removed Podcast functionality, pending (presumably)
complete re-write due to Quicktime being obsolete in newer OS X
versions.

- Created OBCocoa namespace, and associated files src/core/OBCocoa.h/.mm, to
handle OS X - specific system calls. Currently used only by
UBApplication, but can in the future provide a useful interface between
the cocoa framework and OB, to avoid having too much OSX-specific code
in various files

Q_WS_MACX -> Q_OS_OSX
Q_WS_MAC -> Q_OS_OSX
Q_WS_WIN -> Q_OS_WIN
Q_WS_X11 -> Q_OS_LINUX