Basic implementation for the holding tags dressup. Currently only trapecoidal with its two extremes of square and triangular are supported.
I'm not saying it entirely fixes 2751, there are some more UI features and a code cleanup necessary to call that one done.
PR: https://github.com/FreeCAD/FreeCAD/pull/412
issue: http://www.freecadweb.org/tracker/view.php?id=2751