Display dialogs on the primary monitor when no window has focus.

Before this change, we displayed dialogs on the monitor containing the
focused window if there was any, otherwise on monitor 0.  We now use
the primary monitor rather than monitor 0 when no window has focus.

https://bugzilla.gnome.org/show_bug.cgi?id=648305

src/shell-global.c

@@ -1517,8 +1517,7 @@ shell_global_get_focus_monitor (ShellGlobal  *global)
         }
     }
 
-  meta_screen_get_monitor_geometry (screen, 0, &rect);
-  return meta_rectangle_copy (&rect);
+  return shell_global_get_primary_monitor (global);
 }
 
 /**