- 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.

(Solution from previous commit worked only on Linux)

Merging of the new settings and metadata saving feature into the main
development branch

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)

+ clean-up in a few places

Some settings were changed between v1.02 and 1.10 (current), and some
of these changes cause OpenBoard to crash at launch. This commit adds
a function to check for these specific new settings, and wipe the old
values if they are found.
This avoids problems when the user upgrades from 1.02 without deleting
their configuration file.

(This is an alternative to having a post-install script, which would be
ineffective in a multi-user configuration)