Handle video out keys in media-keys

Instead of reimplementing keyboard events capture in xrandr,
add D-Bus methods to rotate the screen, and switch video modes
to the xrandr plugin, and make the media-keys plugin handle call
those methods when the keys are pressed.

This patch almost entirely written by:

Bastien Nocera <hadess@hadess.net>

with minor fixes.

https://bugzilla.gnome.org/show_bug.cgi?id=623223