Adds Pixabay image search, to replace the defunct Google search

also added a function to get slightly larger images (though the Pixabay
API doesn't give width and hight info about these, so it's not currently
used)

- Fixed display
- Fixed/added support for pages
- Added image title generation, so it can be e.g added to OpenBoard
library

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

Search basically works; but just one page of elements are retrieved.
Metadata is assigned ok.
To do: paginate search results; correct display; and later, add filters

- 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