trancode: fix time_base and pkt_sri for audio, other cosmetic updates
global headers: create own AAC/MP4A header only if a valid does not exist
libav muxer: add AVMATROSKA muxer type
Makefile.ffmpeg: enable h264_mp4toannexb bsf for libav muxer
Makefile.ffmpeg: prefer static linking for x264, vpx, vorbis libs to avoid conflicts with distro
AAC fixes (ADTS header) in transcoder and libav muxer
Revert "Makefile.ffmpeg: prefer static linking for x264, vpx, vorbis libs to avoid conflicts with distro"

This reverts commit 8e876995f352b28dc7ca0f5ad683d82e35b38370.

It does not work..
WEBUI: help changes, cosmetics and icon