Turn FFMS2.avsi into real code

[myrsloik]

Simply recreate FFInfo(), FFmpegSource2() and FFImageSource() as functions inside the plugin instead of being in an avsi file. This will be simpler for the users and is only a small amount of work. Celebrate success by also adding FFMS2() as a shorter alias for FFmpegSource2(),

[dwbuiten]

FFCopyrightInfringement().