ClutterGst: Added support to set/unset clutter textures on the fly.

This allows setting/unsetting the video texture where to display when we
need them.
CallWindow: Only create the extended video stream window when required.

Also added a menu item to display it back when the window has been
closed.
VideoOutput: Dropped now useless setup method and GSettings code.

This is now useless as everything is automatic for the user.