allow setting the font size of the Command Window

This was slightly complicated. To make the old text change size, we had
to change its size manually. And to make newly inserted text have the
desired size, we had to write it onto the defined style objects.
scroll to bottom of pane when prompts are displayed

Whenever a new prompt is displayed, the cursor moves to the end.
Also, whenever text is written to the command window or the screen is
cleared, the cursor matches the position it already had, unless it
was moved before the prompt.