meta/window: Make window tile API public
At the moment the tile API is private, this MR makes it public.
Why? We'd like to be able to access this API in Gala (elementary's wm) to add an option in the window contextual menu to tile windows:
Adding this option helps to discover the feature as well as the shortcuts associated with it. GNOME Shell could take advantage of this API as well and do something similar.
More information: https://github.com/elementary/gala/issues/1031