Cleanup and speed improvements

As I’m often using the handy Overlay UFOs extension, I’ve had a look at the code and tried to clean up some things and make the scripts use less resources. Specifically I tried to minimize the time it takes to draw the source fonts.

- Overlays are now drawn behind the outline
- Window is a bit smaller to save space on screen
- Colours are saved in preferences
- updateView is called less often to save computing cycles
- MojoDrawingTools pen and ViewSourceFonts are in separate files so they are cached in compiled form automatically.