Add "window-created" signal to MetaDisplay, "unmanaged" signal for MetaWindow

For some consumers it's significantly more convenient to be able
to directly connect to a signal on the Window to know when
Mutter is done with it, rather than having to connect to each
Workspace object (and handle workspace additions, etc.).

Similarly, add window-created which acts globally.

https://bugzilla.gnome.org/show_bug.cgi?id=598289