Suggestion: some changes in String API

[Tsuda Kageyu]

I suggest to make some small changes in String API.

Currently, String class has some methods which convert itself into a 
standard C/C++ string, but they are somewhat confusing and inconsistent. 
So I'd like to change these methods like this:

// Current
std::string to8Bit(bool unicode = false) const;
const std::wstring &toWString() const;
const char *toCString(bool unicode = false) const;

// New
std::string toStdString(Type t = Latin1) const;
const std::wstring &toStdWString() const;
const char *toCString(String::Type t = Latin1) const;

The actual code is at https://github.com/taglib/taglib/pull/218

I think that toCString() is no longer needed and can be substituted by 
toStdString() so we can remove it.