monitor-manager: Keep pointer to backend

Instead of accessing the global singleton, keep a pointer to the
backend in the MetaMonitorManager struct.

https://bugzilla.gnome.org/show_bug.cgi?id=785381

src/core/keybindings.c

@@ -44,6 +44,7 @@
 #define KEY_GRAVE 0x29 /* assume the use of xf86-input-keyboard */
 #endif
 
+#include "backends/meta-backend-private.h"
 #include "backends/meta-monitor-manager-private.h"
 #include "backends/meta-logical-monitor.h"
 #include "backends/x11/meta-backend-x11.h"