Merge ITextMark and BaseTextMark
Turns out we don't even need two-phase initialization,
by transparently postponing registration until we get
a non-empty file name, either at constuction, or at
file name change times.
Change-Id: I3e87e47c820066e6707e946fc474ab9c1993e61f
Reviewed-by:
Eike Ziller <eike.ziller@digia.com>