Move files part of the playback core to player sub-directory

All these files access mp_core.h and MPContext, and form the actual
player application. They should be all in one place, and separate
from the other sources that are mere utility helpers.

Preparation for splitting mplayer.c into multiple smaller parts.
Split mplayer.c

mplayer.c was a bit too big. Split it into multiple files. I hope the
way it's split makes sense. Maybe some things don't make too much sense,
or go against intuition. These will fixed as soon as I notice them.

Some files are a bit questionable (misc.c, osd.c, configfiles.c), and
suggestions how to organize this better are welcome.

Regressions are possible due to reorganized include statements.
Obviously I didn't just copy mplayer.c's orgy of include statements, but
recreated them for each file. It's easily possible that there are
oversights and mistakes, which will show up on other platforms.

There is one actual change: the public avutil.h include is removed from
encode.h, and I tried to replace most FFMIN/FFMAX/av_clip uses. I
consider using libavutil too much as dangerous, because the set of
include files they recursively pull in is rather arbitrary and is
different between FFmpeg and Libav.