Add documents to public functions except for CODEC specific info extension ones.
Rearrange CODEC identifier definitions in the public header.
box: Move box handling functions.
importer: Remove mp4sys_ prefix for some importers.

Importers itself have nothing to do with MP4 Systems.
Fix mistake of conditional compilation
Merge branch 'plain' into build-msvc

Merge revision: r728
Conflicts:
	description.c
	importer.c
	isom.c
	muxer.c
	write.c