Properly figure out mount stabilisation.

Still a bit hacky since I don't know how to get the magic constants
which are used as values of RC*_FUNCTION, and I don't know how to figure
out which values that '*' might be allowed to take.
Properly determine which RC*_FUNCTION exist.

Also renamed/moved some constants, but don't tell anyone or I'll get
into trouble for doing two unrelated things in a single commit.
Give description that actually describes module.
Allow better colour specification for view region.

Use webcolors module to handle many predefined colour names, and hex
colours.
Draw target icon at the centre of view.

Should allow rough testing of MNT_MODE=GPS_point.