lavfi doxy: add a page for lavfi.
doxy: add a section about versioning.
lavc doxy: add avcodec.h to the libavcodec group.
lavr doxy: add version.h to the lavr group
doxy: don't sort struct members alphabetically

The declaration order is more meanigful in most cases.
lavfi: rename AVFilterFormats.format_count to nb_formats

This is more consistent with naming in the rest of Libav.
af_aformat: cosmetics, fix indentation
vf_format: do not use the AVFilterFormats internals.

Use the formats.h API instead.
lavfi: factorize freeing a link
avfilter.c: cosmetics, reformat
avfilter: check a malloc in avfilter_link().

Additionally change sizeof(type) into sizeof(var)