This reverts commit 08f7b3c1.

This reverts commit 4aa0ee61.

- 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)`

When the time comes to merge dev into master (for real), the
following may be useful:
https://metlos.wordpress.com/2012/01/13/
git-merging-after-a-revert/

Items duplication adapted to new ideology of adding media items on the board.

Have a problem - should take care about saving original src path.

Still have some mesh with undo stack and with widget content.

1st we remove original widget, then we disable undo stack, create a copy of widget and make it a tool. Then undo stack enables back. That widget has tool as parent, so we not need to manage him by scene.

Import PDF: progress bar is not visible

Now coping creates a full copy of already created item. Copy has it's own content (like video, audio or folder) and stores original item properties (like locked, pos, size, freezed e.t.c.).