af: add metadata field to af_stream and af_instance

This allows to propagate metadata information to audio filters.

Closes #632
af_volume: add replaygain support

This adds the options replaygain-track and replaygain-album. If either is set,
the replaygain track or album gain will be automatically read from the track
metadata and the volume adjusted accordingly.

This only supports reading REPLAYGAIN_(TRACK|ALBUM)_GAIN tags. Other formats
like LAME's info header would probably require support from libav.
af_volume: add support for replaygain pre-amp and clipping prevention
command: set 'media-title' property for bluray disc with meta-data
command: fix wrong condition & remove redundant stream type checking
player/main: Fix Cygwin build

Xlib.h (included from x11_common.h) defines a macro 'Status' as 'int'.
This messed up a bunch of definitions in windows.h and broke the build
in Cygwin. Including windows.h first seems to solve the problem.

This commit also removes the definition of an unused flag.