MetaWindowActor: fix reference counting issue

We need to use g_signal_connect_object(), rather than g_signal_connect(),
because the window actor can be destroyed before the window emits
the final notify::appears-focused inside unmanage, if the plugin
decides that it doesn't want to animate the destruction (which
happens with dialogs and the default plugin)

https://bugzilla.gnome.org/show_bug.cgi?id=706207