ShellWindowTracker: fix reference counting of ShellApp

All get_app_from_*() helpers are transfer full, but
get_app_from_gapplication_id() was directly returning the result
of lookup_app(), which is transfer none.

https://bugzilla.gnome.org/show_bug.cgi?id=721439
ShellWindowTracker: remove gtk-doc marks from private functions

static internal functions should be documented with /*, not /**

https://bugzilla.gnome.org/show_bug.cgi?id=721439