shell-app: Raise windows in reverse order to preserve the stacking

shell-app: Raise windows in reverse order to preserve the stacking

ShellApp keeps track of windows sorted by most recently used first
which means that when raising windows we need to start from the end of
the list to finish with the correct stacking order.

This patch just makes the code agree with the comment which was
already there.

https://bugzilla.gnome.org/show_bug.cgi?id=676371