Small fix with backspace and preedit

If we are still inside the preedit string then remember to update the
preedit string on each backspace.
Small fix for backspace and surrounding text
Try to continue editing any word under the cursor

Fixes https://github.com/lewtds/ibus-ringo/issues/12
Deprecate focus_tracker.py
Improve is_processable_key()

We now only accept a-Z and all keys defined in the current input method
(Telex/VNI, etc.).
The BaseBackend should not update_composition() on backspace

That should be the job of the inheriting class.
Deprecate direct_backend.py