Re-factoring AA tree code from m_misc.c/m_misc.h into its own files

What the title says. The AA tree-related code now lives in the files m_aatree.c and m_aatree.h. Part of why I did this was to solve this m_misc.h/w_wad.h cyclic dependency problem (involving MAX_WADPATH and AA trees themselves) mentioned in the now-removed comments, another reason was ...only OpenGL uses AA trees at all, why include the relevent structs/functions/otherwise anywhere except where is necessary (which is very few files as it turns out)?

Otherwise, it just looked better on its own rather than mixed with all the other stuff already in the m_misc files. Not really important or anything affecting gameplay at all I guess.