Bug 719639 - @xrefitem with empty heading string

Adjusted documentation and in case the title is the empty string in a DocXRefItem the visitPre and visitPost do nothing.
Bug 719639 - @xrefitem with empty heading string

Replaced:
  if (!(x->title().length())) return;
by:
  if (x->title().isEmpty()) return;
Merge pull request #61 from albert-github/feature/bug_719639

Bug 719639 - @xrefitem with empty heading string