generation of SA-MP functions in 'sampgdk' namespace (C++ only)

Those `#define` aliases are messing my code up (and I generally think `#define`'s shouldn't be used, at least in C++ code), so I moved them into the `sampgdk` namespace. There are some further comments regarding some of the commits. I'm not sure if this is a change which should be made available for everyone (because possible backward compatibility problems), but I thought I just make those changes and share them here to make you that decision a little bit easier.
Other than that the changes I made didn't seem to affect Doxygen in any way (neither good nor bad).