Abstract daemon and add `mutter` prefix (split from !139)

This MR is a split from !139 (merged), it abstracts daemon class to allow different behaviours and adds mutter prefix to mutter dbus proxies.

The standard behaviour is the only one implemented and is called daemon-user.

There are some functions that aren't used yet, but will be used on new daemon types (If you think those changes should be moved when used, I'll change them).
These are grd_daemon_get_cancellable and grd_daemon_get_rdp_server.
Moved.