Rework the UI

This branch is an experiment in reworking the UI. The proposed process is:

- [x] Provide a single oracle command that opens the oracle panel
- [x] Run the `what` mode on the current cursor position
- [x] Populate a selection menu with the available modes for that cursor position
- [x] Default to running `describe` on the current position
- [x] Select box triggers running a different mode
- [ ] Provide a way to re-run the oracle at a different cursor position
- [ ] Purty it up

Re: the last point, we probably don't want to run the oracle just by moving in the editor as movement may be triggered by clicking through the oracle output.