ffmpeg transition

[Harald Sitter]

wily is moving from libav back to libffmpeg. library-wise we seem to
be sorted, but there are also runtime utils to be considered here.

to transit them libav-tools dependency/recommends/suggests needs
changing to ffmpeg and the runtime tool that is being called probably
needs to be changed to ffmpeg as well

rdeps:
- soundkonverter
- kphotoalbum
- kdenlive

I also seem to recall amarok and k3b using some runtime tools from
libav so perhaps have a look at those as well.

HS