Form fields and BFU text-input widgets then convert from UCS-4 to UTF-8.
If not all UTF-8 bytes fit, they don't insert anything. Thus it is no
longer possible to get invalid UTF-8 by hitting the length limit.
It is unclear to me which charset is supposed to be used for strings
in internal buffers. I made BFU insert UTF-8 whenever CONFIG_UTF_8,
but form fields use the charset of the terminal; that may have to be
changed.
As a side effect, this change should solve bug 782, because
term_send_ucs no longer encodes in UTF-8 if CONFIG_UTF_8 is defined.
I think the UTF-8 and codepage encoding calls I added are safe, too.
A similar bug may still surface somewhere else, but 782 could be
closed for now.
This change also lays the foundation for binding actions to non-ASCII
keys, but the keystroke name parser doesn't yet support that.
The CONFIG_UTF_8 mode does not currently support non-ASCII characters
in hot keys, either.
