rdp-layout-manager: Recreate mirror-primary streams when streams close

Quoting the final commit message here:

When using gnome-remote-desktop in a virtual machine, and the screen is
resized in a mirror-primary stream, the session is gone, since mutter
closes the stream, and since gnome-remote-desktop closes the session,
since the stream is gone.
To handle this situation better, extend the layout manager to recreate
the layout, to rerecord the primary monitor again.

This MR reworks the internal stream and surface handling in the RDP backend.
Basically, the session won't be tied to one RDP surface and stream any more (multi-monitor is still not implemented yet, but these changes will be essential for multi-monitor regardless).
Also, stream creation now involves an additional id, which makes it easier to look up respective settings, when a stream has been created.

Built on top of: !171 (merged) (commits included here)
Depends on: mutter!2911 (merged)
Should be merged together with: mutter!2911 (merged)