Support undoing previous words with backspace

Now that process_sequence() has acquired support for separators, we don't need to reset() on unaccepted chars. We just keep them in the raw sequence and the committed string. So they can get pretty long like this: 

- stored key sequence = 'con meof ddieen'
- committed string = 'con mèo điên'

As such, it is now even possible to undo the grave tone in mèo. To test, type in 'con meof ddieen' and repeatedly press backspace so that the cursor is right after mèo. Press backspace one more time, you can see that the grave tone is undone and we have 'meo'.