command: add filtered-metadata property

Goes in hand with the previous commit; the main purpose is to easily
show on OSD what is shown on terminal.
demux_mf: move mf.c contents to demux_mf.c

There's no reason why parts of this demuxer would be in a separate
source file. The existence of this code is already somewhat questionable
anyway, so it may as well be dumped into a single file.

Even stranger that demux.c included mf.h for no reason (it was an
artifact from 2002 when the architecture was uncleaner).
demux_mkv: reduce log noise

This message can happen a lot for mkv files which index clusters in the
seekhead (which is also broken non-sense, but that's a different story).

Also remove a duplicate define from matroska.h.