Streamlined thread message passing

[jon-axon]

Current v0.1 GUI code has functional message passing implemented for communication between the UI and back-end threads, however this is very much proof-of-concept code, and should be reviewed and refined. In particular:

• Can the number of channels be reduced (currently 3 are used)?
• Can the messages passed be made more robust (e.g. inclusion of new types rather than primitives)?

[jon-axon]

Channels reduced from 3 to 2 (one Chipolata -> UI, and one UI -> Chipolata), with new enums to differentiate different messages types sent on these channels.