BTextEncoding: even more style fixes.

BTextEncoding: fix naming and move to BPrivate namespace.

Style fix in Locale Kit headers.

As pointed out by Axel, the wrong style was used in these headers we
inherited from OpenTracker days.

TextEncoding: Style fixes to header.

As pointed out by Axel.

TextEncoding: add Encode/Decode methods.

This replaces convert_from/to_utf8 in a way that doesn't leak memory.

Use the new method in StyledEdit.

Fixes #6252, #3065.

Add text encoding detection to the Locale Kit.

As usual we ask ICU to do the actual work. The TextEncoding constructor
is fed with a sample of the text to identify (ICU docs recommend a few
hundred bytes). The text is analyzed in various ways (bytes patterns
such as UTF-8 escaping schemes, common letter sequences from known
languages, byte order marks) and an encoding is determined.

Replace code in StyledEdit by this new implementation.

Note that ICU seems to always return some valid encoding, even with fed
with obviously non-text data. This makes StyledEdit open the files no
matter what, where it would error out before.

Fixes #9395.