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.
rotation: Remove unneeded Clutter dependency

There's no need for a direct clutter dependency, we use the video
widget's helper functions now.
properties: Use set_accels_for_action()

Instead of add_accelerator() and remove_accelerator().
save-file: Use set_accels_for_action()

Instead of add_accelerator() and remove_accelerator().
screenshot: Use set_accels_for_action()

Instead of add_accelerator() and remove_accelerator().
main: Use set_accels_for_action()

Instead of add_accelerator() and remove_accelerator().
save-file: Handle "Save" multimedia key
properties: Handle "View" remote key
main: Remove properties accel in main

It's already setup in the properties plugin.
data: Remove dead web browser plugin data files
main: Remove GtkUIManager and related usage in playlist

The playlist is an internal widget, so crudely replace the popup menu
with buttons.
main: Remove unused totem_object_get_ui_manager()
main: Remove unused clear_playlist_action_callback()
main: Remove dead GtkActionGroups

Those were ported to GActions some time ago.
main: Remove GtkUIManager usage

Plenty of deprecated code gone.
main: Remove RTL helpers

The code now lives directly in GTK+'s GtkIconTheme handling.