MpaDecFilter: Fix MPC-HC compilation with ffmpeg audio disabled

Lite builds were unaffected, but if you built a "normal" MPC-HC
with ffmpeg audio decoders disabled, it would not find
m_FFAudioDec, as it is only around when ffmpeg audio decoders are
enabled.
FFAudioDecoder: fix audio decoding for new ffmpeg updates.
Updated Catalan translation.

Refs: #2408.
Some testing on SetAlwaysOnTop. (not working)
PPageExternalFilters: cosmetics.

* Use more meaningful functions names.
* Use correct visibility for the members variables and functions.
Try to fix a crash in RegisterHotkeys.

This commit fixes #2971.