Undo and delete for token input don't behave as expected

[David Aspinall]

Ordinary undo with token input does not behave as the user might expect (certainly not as X-Symbol once did): it removes the last character of input in the expanded token sequence. We might be able to improve this by adding a remapping for the undo function.

We should also document useful ways of editing with the token input (Quail mode) enabled).

Similarly, the delete key doesn't behave as expected after token input, deleting the last character of the token.

[David Aspinall]

Fixed, for now. Added some key bindings to delete tokens and undo token insertion in a single step, but once token input is complete (space or another character is typed). Still probably not quite ideal behaviour, but an improvement. Interaction with Quail is tricky here.