altTab: Don't mess up MRU order while cycling windows

Commit bd6e7f14 reimplemented the cycle keybindings to
fix cycling between more than two windows, but the approach
of highlighting cycled windows by actually focusing them has
the drawback that cycling messes up the MRU order of windows.
To fix this, only change the window focus when the operation
finishes, and use a dedicated actor that draws a border around
a window clone for highlighting.

https://bugzilla.gnome.org/show_bug.cgi?id=771063