backend: Reset loaded text subtitles when closing video

https://bugzilla.gnome.org/show_bug.cgi?id=728339
backend: Add a way to rotate without animations
backend: Automatically rotate mobile phone videos

For certain types of videos, and given a new enough version of
the "qtdemux" plugin, GStreamer will tell us which is the best
rotation for a video. This will automatically show videos the
right way around for videos captured upside-down or in portrait mode
on smartphones.