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

NOTE: Patch copied from mutter and adapted for metacity. This adds
only description in doc/theme-format.txt as all other changes has
been added already.

doc/theme-format.txt

@@ -28,6 +28,10 @@ New Features in Theme Format Version 3.3
 Add two additional button background functions - left_single_background and
 right_single_background - for button groups with just a single button.
 
+There are now additional frame states to style left/right tiled windows
+differently ("tiled_left", "tiled_right", "tiled_left_and_shaded",
+"tiled_right_and_shaded")
+
 New Features in Theme Format Version 3.2
 ========================================