window: Cache _NET_WM_ICON_GEOMETRY

Rather than doing a server round trip each time when retrieving the
icon geometry, use the existing property mechanism to cache it.

https://bugzilla.gnome.org/show_bug.cgi?id=692997

src/core/window-private.h

@@ -220,6 +220,9 @@ struct _MetaWindow
   
   /* whether net_wm_user_time has been set yet */
   guint net_wm_user_time_set : 1;
+
+  /* whether net_wm_icon_geometry has been set */
+  guint icon_geometry_set : 1;
   
   /* These are the flags from WM_PROTOCOLS */
   guint take_focus : 1;
@@ -391,6 +394,9 @@ struct _MetaWindow
    */
   MetaRectangle user_rect;
   
+  /* Cached net_wm_icon_geometry */
+  MetaRectangle icon_geometry;
+
   /* Requested geometry */
   int border_width;
   /* x/y/w/h here get filled with ConfigureRequest values */