VIX-185: initial pass, changing 'channel' to 'element' and 'output' to 'controller' in GUI display only.
VIX-185: change all internal uses of "channel" to "element" within the software.

Serialization stuff still uses "Channel", as that's what the files on disk expect to have in them; this will need to be handled in a separate migration later.

(Controller terms still use "output" instead of "channel". This is still to be fixed.)