Colorsfix - 3 options for syntax highlighter

fix issue #419 and issue #250 and recent report to mailing list

this introduces a preferences window with three different choices for the syntax highlighter

1. Turn it off (it will not turn off the error highlighting though)
2. Make it bright colors (suitable for a dark background, which can be set by the operating system)
3. Make it dark colors (suitable for a light background, which can be set by the operating system)

Preferences are saved and set as appropriate using the QSettings system.