Include file cleanup

[Nicolas Alvarez]

I just did some cleanup of header files. Many were including more than they
should, some were including less and relying on other headers including
what they needed...

For forward declarations, I used 'class' on all of them. But VC++ complains
if an object is forward-declared as 'class' and then there is a full
declaration with 'struct'. So someone with VC++ please fix those
warnings :)

I think DerMeister once fixed one of those warnings by changing the full
declaration from a struct to a class (adding an explicit "public:"). This
may be a good idea; make them all classes for consistency. Unless they
really are "value objects" like USER_MESSAGE (a pair of strings).