Revise/Simplify old optional text rendering
Set things up a bit for selected text rendering
Add MacTextWindow::clearText and MacTextWindow::setSelection
Add MacTextWindow::appendText variant that accepts a MacFont
Currently the font specified in the constructor is not
being used, I explain why in gui.cpp, line 182.
Use the 2 defines in gui.h to specify whether:
1) The new text rendering in general is used
(USE_NEW_TEXT_RENDERER)
2) The MacTextWindow class is used
for rendering (USE_MACTEXTWINDOW)