Finish organizing header definitions (for now) and copying from older headers.
fxcg/rtc.h added for RTC syscalls and fxcg/registers.h added for a list of registers that may be useful (seemed clean).
There are still some header files from the miniSDK on the include folder, out of fxcg/, which don't fit in style with libfxcg but are still present probably for compatibility with ancient code.
These headers provide prototypes for things like ListView and StrView which belong in miniSDK and which we don't have on libfxcg (and I believe it's out of the project scope to have, at least in their present form). Perhaps they should be removed and minicompat dropped forever?