Make input methods support great again

Make input methods support great again

This unbreaks input methods by splitting the part of the code that
does the actual drawing to a separate paintPreeditText() method which
is called from paintEvent().

The proper solution would have been to introduce the preedit string in
the Row object, like is done for completion, but this is too complex
to do at this point.

The only change in behavior is that now the commit string is inserted
in one fell swoop, intead of emulating a number of key events.