Format date- and tag-based tree entry item labels cleanly when the entry has no subject.
Format date- and tag-based tree entry item labels cleanly when the
entry has no subject.

* lib/tk_main.py
  (TKEventTree._ItemLabel): New.
  (TKEventTree.EntryChangedListener): Use new _ItemLabel() helper.

* lib/tk_main.py
  (TKEventTagTree._ItemLabel): New.
  (TKEventTagTree.EntryChangedListener): Use new self._ItemLabel() helper.
Isendir/thotkeeper,cmpilato/thotkeeper