removed references to youtube from videoPicker widget (youtube embedding doesn't work, won't be fixed soon)

e.g 1.3.0-rc.1, or 1.3.0

Also, changed version number to 1.3.0-rc1

- Display a meaningful error message to the user when adding or trying
to play a video
- Make sure the video placeholder is visible even after switching pages

(Not doing so resulted in either a trashed item being selected when
returning to board mode, or a crash if the item had been permanently
deleted)

The previous solution didn't really have any effect; only a handful of
settings were in the mUserSettings or mAppSettings at the moment it was
called. The better solution is to just call value() in the constructor
of UBSetting, which means the setting is cached as soon as it is
created.

- Sort by Z before duplicating a selection (so that relative Z's are
kept)
- in UBZLayerController, iterate only through UBGraphicsScene's fast
access items, not QGraphicsScene::items

- Defined constants in UBSettings for highlighter preview circle colors
- Switch between light and dark colors when changing backgrounds
- For eraser: use color settings that were defined in UBSettings (they
were ignored since the preview circle was added)

When a video is first loaded (placed on the scene), we play/pause it to
load the first frame; but this was also called when the video was
manually stopped. To avoid this, a mStopped attribute was added to
UBGraphicsMediaItem. It is set to true only when the video is stopped by
the user.