Squash more "may be uninitialized" warnings
audio.cpp: Split up into smaller parts
Use macro from util.h for static array size
filesystem.cpp: Split out RGSSAD parts
RGSSAD: Add RGSS2 archive reader

Exactly the same as RGSS1 but different file extension.
Also remove some redundant strings.
RGSSAD: Add RGSS3 archive reader

Everything except the entry table parsing is the same as RGSS1.
Minor cleanups
MRI-Binding: Allow manually specifying load paths in config