ffmpeg plugin: improve encoding format negociation

The ffmpeg encoders are restricted on which raw formats they accept
(each with different constraints). When asked to encode something, the
ffmpeg encoder now checks the codec capabilities and if the suggested
input format does not match, modifies it to something the codec can
actually handle.

Enable "experimental" codecs, as this is needed for AAC support in ffmpeg 0.10.

This allows format negociation to work properly, and as a result we can
enable several new audio formats: AAC (for MPEG family only), AC3, and
raw audio. Declare MKV as B_ANY_FORMAT_FAMILY since it is designed to be
a generic container (so it is possible to put AAC in MKV). Also remove
duplicate entries for AC3.
MediaConverter: let the encoder make changes to the format

If the encoder decides to change the media format, we need to tell the
encoder as well so it uses the same format.
Move the "raw audio" format to the WAV format family.

It doesn't work well in AVI and MPEG.