Handling of wide characters is incomplete

[osa1]

A wide character takes more than one column on terminals. Examples: the tab character (8 cols), Ｈ (2 cols).

Currently in the input widget we don't handle these so if I enter wide characters the characters after that are rendered incorrectly, and glitches occur as I move the cursor.

I haven't checked, but I suspect message area should also have the same problem.

In termbox we use unicode-width to determine width of a character. However termbox does not implement wrapping so it's TUI's job to implement splitting the line at the right point in the presence of wide characters.