Add remote access controller API

Add API to let GNOME Shell have the ability to get notified about remote access sessions (remote desktop, remote control and screen cast), and with a way to close them.

This is done by adding an abstraction above the remote desktop and screen cast session objects, to avoid exposing their objects to outside of mutter. Doing that would result in external parts holding references to the objects, complicating their lifetimes. By using separate wrapper objects, we avoid this issue all together.