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 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.

closes #4860