Properly differntiate Dot- and Color groups

[markus-k]

Dot Groups (user-selectable) and Color Groups (fixed current sinks) are currently a bit mixed up.

• DotGroup should have the None option removed and set_dot_groups should take an Option<DotGroup> instead.
• The Group enum should be renamed to ColorGroup, and possibly their items renamed to Red, Green, Blue (altough there is no electrical reason these have to be these specific colors, which could be confusing).
• set_group_current should be renamed to set_color_group_current.
• set_group_brightness should take a DotGroup and be renamed to set_dot_group_brightness.

Thses changes should make the differences between fixed color groups and user selectable dot groups clearer, as the datasheet itself it not always super clear on that end.