[Lazarus] "Character" -> "Codepoint" in LazUTF8 function names

[Juha Manninen]

I changed "Character" to "Codepoint" in some LazUTF8 function names in
r56692. Please take a look.
"Codepoint" is more accurate and hopefully reduces confusion.
It was discussed in forum and I agree with the importance of using
correct terms.
 http://forum.lazarus.freepascal.org/index.php/topic,39045.msg268541.html#msg268541
With Unicode the term "Character" is very ambiguous.

Existing code that uses those functions now throws "deprecated" warnings.
Sorry for the inconvenience.

Juha