monitor-manager: Store logical monitors in a GList

Instead of storing the logical monitors in an array and having users
either look up them in the array given an index or iterate using
indices, put it in a GList, and use GList iterators when iterating and
alternative API where array indices were previously used.

This allows for more liberty regarding the type of the logical monitor.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/core/workspace-private.h

@@ -56,6 +56,7 @@ struct _MetaWorkspace
 
   MetaRectangle work_area_screen;
   MetaRectangle *work_area_monitor;
+  gint n_work_area_monitor;
   GList  *screen_region;
   GList  **monitor_region;
   gint n_monitor_regions;