Bring msGetEncodedString() and msConvertWideStringToUTF8() closer to each other

[dmorissette]

In ticket #2237, msConvertWideStringToUTF8() has been created to do a similar job to msGetEncodedString(), but working on a wchar_t* input string instead of char*.

At the moment both functions do a similar task but have very different names and implementations.

In 5.2 we need to bring both functions closer to each other:

1- Rename msGetEncodedString() to msConvertStringToUTF8()

2- Use a similar implementation for both functions, possibly by trying to keep the best of both.