ctrlAltTab: Fix external DOCK windows

We always add external DOCK windows to the ctrl-alt-tab switcher,
e.g. separate dock applications or nautilus' desktop windows.
Since commit 1f46a0dc, all items in the switcher are expected
to set a proxy parameter, but the aforementioned code was not
updated accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=695395