Move helper functions out of common utility for better locality

[mthrok]

This commits move helper functions/definitions around so that better locality of logics are achieved.

Detail

ffmpeg.[h|cpp] implements classes that convert FFmpeg structures into RAII semantics.
Initially it these classes included the construction logic in their constructors, but such logics were
extracted to factory functions in #2373.

Now the reason why the factory functions stayed in ffmpeg.[h|cpp] was because the logic for
the initialization and clean-up of AVDictionary class was only available in ffmpeg.cpp.

Now AVDictionary class handling is properly defined in #2507, the factory functions, which are not
that reusable better stay with the implementation that use them.

This makes ffmpeg.h lean and clean, makes it easier to see what can be reused.

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Hey @mthrok.
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