Fix gcc "deprecated conversion from string constant" warnings

[jng]

The Linux build of MapGuide is very noisy, spamming a lot of "deprecated conversion from string constant to char/wchar_t" warnings.

The main source of this warning spam is from MgObject and the DECLARE_CLASSNAME macro that code-gens this implementation for derived classes:

The declaration

virtual char* GetMultiByteClassName();

Should be

virtual const char* GetMultiByteClassName() const;

And the declaration

virtual char* GetNameSpace();

Should be

virtual const char* GetNameSpace() const;