This commit adds basic screen casting and remote desktoping
functionalty. This works by exposing two D-Bus API services:
org.gnome.Mutter.ScreenCast and org.gnome.Mutter.RemoteDesktop.

The remote desktop API is used to create remote desktop sessions. For
each session, a D-Bus object is created, and an application can manage
the session by sending messages to the session object. A remote desktop
session the user to emit input events using the D-Bus methods on the
session object. To get framebuffer content, the application should
create an associated screen cast session.

The screen cast API is used to create screen cast sessions. One can so
far either create stand-alone screen cast sessions, or a screen cast
session associated with a remote desktop session. A remote desktop
associated screen cast session is managed by the remote desktop session.

So far only remote desktop managed screen cast sessions are implemented.

Each screen cast session may have one ...