messageTray: Use monitor geometry for dwelling

messageTray: Use monitor geometry for dwelling

In multi-monitor setups, the screen might extend below the edge of
the monitor that holds the tray. In that case the tray is currently
triggered from a secondary monitor, which is rather surprising;
change the check to use the correct monitor geometry instead.

https://bugzilla.gnome.org/show_bug.cgi?id=683044