A per-window _MUTTER_HINTS property allowing plugins to use custom hints. The
property holds a colon separated list of key=value pairs; plugin-specific keys
must be suitably namespaced, while 'mutter-' prefix is reserved for internal
Mutter use only.

This commit adds MetaWindow::mutter-hints property, and
meta_window_get_mutter_hints() accessor, as well as the internal machinery for
reading and updating of the hints.

https://bugzilla.gnome.org/show_bug.cgi?id=613123