Fix UnicodeWarning when searching for completion choices

Typing non-ascii characters (German Umlauts, in my example) into
the `task_entry` text box throws me a UnicodeWarning:

    ./src/gtimelog/main.py:482: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
      if entry not in [row[0] for row in self.completion_choices]:

Related bugs in PyGTK:
- https://bugzilla.gnome.org/show_bug.cgi?id=323901
- https://bugzilla.gnome.org/show_bug.cgi?id=314560