monitor: Don't get the monitor manager from the backend

We will both create and destroy monitors during initialization (when
using the X11 backend), so don't try to access the monitor manager from
the backend, but store a pointer to it instead.

It's stored in MetaMonitor even though only MetaMonitorTiled uses it,
mostly because it makes more sense to store such a pointer there.

https://bugzilla.gnome.org/show_bug.cgi?id=781723