shell: Don't use workarea height

Instead of monitor height to check for the available height, as
the workarea height won't have been updated fast enough after
a monitors-changed signal.

https://bugzilla.gnome.org/show_bug.cgi?id=673869#c47

shell/gnome-control-center.c

@@ -1151,7 +1151,8 @@ get_monitor_height (GnomeControlCenter *self)
   GdkScreen *screen;
   GdkRectangle rect;
 
-  /* FIXME use workarea instead of raw screen geometry */
+  /* We cannot use workarea here, as this wouldn't
+   * be updated when we read it after a monitors-changed signal */
   screen = gtk_widget_get_screen (self->priv->window);
   gdk_screen_get_monitor_geometry (screen, self->priv->monitor_num, &rect);