band-aid for completion

Unlike @Carreau's attempt at using proper CodeMirror completion,
this is just a band-aid to get the existing completion back to working for 2.0.

This adds a keypress handler on the completer,
which then calls insert with the charCode of the key press event,
replacing the final `elif` branch of the keydown handler.

This cannot be done with keydown,
since keydown doesn't know what character is incoming,
only the hardware key that is struck.
Merge pull request #5112 from minrk/completion-band-aid

Band-aid for completion in the notebook: not the ideal solution, but will have to do the job for now so that typing while completing works.

closes #4860