appDisplay: Move focus to popup only when necessary

Unlike for the main app view, where we only move the key focus once the
users starts navigating, the key focus is moved immediately when opening
a folder popup. This is unexpected, so make app folders consistent with
the main view.
As arrow keys will not work while the container itself has key focus, we
handle those explicitly by translating them to TAB_FORWARD and
TAB_BACKWARD respectively.

https://bugzilla.gnome.org/show_bug.cgi?id=731477