theme: Add tiled_left/tiled_right frame states

It may be desirable for theme authors to treat side-by-side tiled
windows differently, for instance to give the edge-touching border
a width of 0, so add additional frame states for tiled windows.

https://bugzilla.gnome.org/show_bug.cgi?id=637330

src/core/frame.c

@@ -291,6 +291,12 @@ meta_frame_get_flags (MetaFrame *frame)
   if (META_WINDOW_MAXIMIZED (frame->window))
     flags |= META_FRAME_MAXIMIZED;
 
+  if (META_WINDOW_TILED_LEFT (frame->window))
+    flags |= META_FRAME_TILED_LEFT;
+
+  if (META_WINDOW_TILED_RIGHT (frame->window))
+    flags |= META_FRAME_TILED_RIGHT;
+
   if (frame->window->fullscreen)
     flags |= META_FRAME_FULLSCREEN;