This is essentially a combo widget containing a single-line GTextEditor
and two buttons for increment and decrement. The GTextEditor::on_change
callback is hooked to prevent non-numeric input but it's not entirely
perfect since that callback is asynchronous. This will work until we have
some more sophisticated input validation mechanism though.
I added focus rects to these widgets because I had just started working on
focus support and I was excited but it doesn't really make sense for these
things to have focus rects. :^)
While I was here I also optimized the repaint code to only update the edited
glyph in the glyph map when editing its pixels.
