Use the dispatcher for communication, instead of callbacks.

[nein09]

Relative to the last few sets of changes, this is small.

I think this is the last part of the refactor, unless @aeschylus and @iangilman can think of other things they'd like to see split out or modified.

[iangilman]

Looks good to me, but I'll let @aeschylus review and merge.

We can save any further refactoring to be done as part of the remaining bug and feature work as needed. You've created a good structure to support it.