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)

(Reading is still done synchronously, for now at least)

- 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)`

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

persistence of not required media on disk. Second step updated the cleanSource methods in classes that handle files on disk

Document navigator: "Add pages from file" isn't work with old UBZ

Show Desktop: "Erase all" button isn't work