Repositioned window in activities

We need to update the clone position if window size changed, plus rename meta window 'size-changed' callback accordingly.

Depends on !4 (closed), fixes #3 (closed) (https://bugzilla.gnome.org/show_bug.cgi?id=776588).

Imported from https://bugzilla.gnome.org/show_bug.cgi?id=792681