Recognize CMD-Up and CMD-Down as alternatives to Home and End on OS X.
Set QFont properties to help guarantee a monospace default source font.

 * If the user configured a source font via a config file, then do not
   attempt to make the font monospace.

 * Use the first of these methods that produces a fixed-pitch font
   according to QFontInfo:
    - QFont::setStyleHint(QFont::Monospace)    [Qt 4.7+ only]
    - QFont::setStyleHint(QFont::Courier)
    - QFont::setFixedPitch(true)