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.