window: Add a META_MAXIMIZE_BOTH maximization flag

Which has both directions set. This is a simple cleanup to
remove the constant VERTICAL | HORIZONTAL all over the place.

https://bugzilla.gnome.org/show_bug.cgi?id=726352
window: Export a bunch of state flags as accessor methods

This is necessary to reimplement window menus in gnome-shell.

https://bugzilla.gnome.org/show_bug.cgi?id=726352