gui2/ttitle_screen: Place the version/revision number label on the bottom left corner of the screen

Finally achieved this in a resolution-safe manner (unlike r47156) by
moving the label to a separate layer of the parent stacked widget that
goes beneath the main button-littered layer (to avoid stealing events).

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56482 75b3775d-3801-0410-9e12-e2e43121521d