Remove meta_window_new_with_attrs

The compositor code used to handle X windows that didn't have a
corresponding MetaWindow (see commit d538690b), which is why the
attribute query is separated.

As that doesn't happen any more, we can clean up. No functional changes.

Suggested by Owen Taylor.

https://bugzilla.gnome.org/show_bug.cgi?id=721345
Discourage server grabs

Server grabs are not as evil as you might expect, but there is agreement
in that their usage should be limited.

Server grabs can cause things to go rather wrong when mutter emits
a signal while it has grabbed the server. If the receiver of that signal
waits for a synchronous action performed by another client, then you
have a deadlock. This happens with Mali binary GLESv2 drivers :(

https://bugzilla.gnome.org/show_bug.cgi?id=721345