Render line numbers using TextAreaPainter

Fix the line number font rendering problem on the Mac.

Now, the TextAreaLineNumbers class extends the TextAreaPainter and just
overloads the paintLine method to make it render the line number instead
of rendering the whole line. Other than that, the superclass' drawing
code is used.

So: the same drawing code that renders the source code text in the
editor is now also being used to render the line numbers.