altTab: Always activate MRU window when activating an app

Commit 2499f2ed went back to using shell_app_activate() for
selecting an app, which favors windows on the current workspace;
this is the behavior we want for instance when activating a
launcher, but it's wrong for the alt-tab list - explicitly
request the first (e.g. MRU) window in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=700356