Simplify the settings

There are only two choices possible for the full/half-width characters,
so a combobox is a bit overkill.

Also, it's not about punctuation only, but also numbers, letters,
symbols,...

We have a good default, so we might as well use a simple checkbox.
Reorganize some code

This is just to make the next commit easier to review.
Implement full-/half-width support

Wan Leung recently added `CangJie::getFullWidthChar` to libcangjie, so
we should use it.

This respects the user settings.

fixes #9