The documents' metadata.rdf file is now persisted only when a scene in
the document is also persisted; as well as when the document is modified
(trashed / path changed) in the the Documents pane.

Code was cleaned-up a bit too (added a forgotten return value, etc)

+ clean-up in a few places

- 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)`

- 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.

Presumably due to the change in how pen strokes were saved to file (commit
8ed2e245), pen strokes with pressure levels were badly saved. They were
converted to lines instead of polygons, meaning that every time a page
was saved or duplicated, the lines would look worse and worse -- and
artefacts would appear.

This should now be fixed.

Page DPI is now saved as the DPI that was read when opening the file.
While not a perfectly fool-proof fix, it will at least allow files to be
migrated between OB 1.02 and 1.10.

- Added an overload for setMatrix in UBGraphicsMediaItem, to propagate
  matrix changes to the child videoItem
- Upon loading a video, the child videoItem is now added correctly, and
  set to the right position

The lines removed in this commit led to (presumably) unintended
behavior: when saving a document, pen strokes were saved not as a group,
but just as their constituent polygons.
This meant that the following block of code, that handles saving pen
strokes correctly, was never executed.

This doesn't fix the z-Value issue (#12), but it seems to be a step in the right
direction.

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

Threaded the storage of the scene. Asynchronous reading of the previous and next scene to be stored on cache.