This fixes an issue where if one document was imported with a different
DPI than the current one, any document created thereafter would have
this same value (which could then cause problems if a PDF was added to
that new document).
Saving this value to UBDocumentProxy not only makes more sense, it also
fixes this issue.
- 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)`