1.- Fix mouse event handling. Some weirdness happens when selecting
    text with the mouse and it moves to the margins. This behaviour
	seems to be the same as in Windows, though.

2.- Improve speed. There are two main issues to consider
    a) Improve drawing speed. Profiling is needed, and avoid using pango
    so heavely. A posible solution is to simplify the calculations when
    the font is monospaced, or, even, allow only monospaced fonts.
    b) Avoid redrawing the entire screen when possible. The redrawing code
    is too pessimistic and that slows things
    down too much. This has to do with the following point

3.- Fix the invalidation calculations. In TextArea.cs there is a Invalidate
    method that gets called when a redraw is needed. I don't understand the
    coordinates received (seen by tracing them), so it's necessary to
    understand them and fix the method, fix the calculations on origin 
    (better) or both.

