Because we were sorting the Alt+Tab list by user_time rather than
stacking order / MRU, it was possible for the currently-focused window
to sometimes not be the first app in the list. Fix this by using
meta_display_get_tab_list() to get the proper MRU ordering of windows
on the current workspace, and then convert that to an ordered list of
apps.
https://bugzilla.gnome.org/show_bug.cgi?id=645026