better doxygen support

When I generated the documentation from asss 1.4.4 via doxygen, I noticed that many things were missing from the generated output. For example, only 16 callbacks were listed, primarily from game.h. Out of the some 40+ .h files, only 18 of them supported doxygen.

Although I hadn't really used doxygen before, I find that it has the potential to be a very useful and beneficial tool for not only documentation purposes, but also for quick lookups and the such.