emms-source-file.el: emms-source-file-directory-hint

[Yoni Rabkin]

`emms-source-file-default-directory' is currently the only completion
for calling `emms-add-...'.

My recent patch will replace `emms-source-file-default-directory' as the
completion with the directory of the track at point if:

    * we are in a playlist buffer

    * there is a track under point, and that track is a file

    * the new variable `emms-source-file-directory-hint-p' is t (now the
      default)

I hope the reasoning behind it is clear: playlists probably contain
related tracks, and related tracks are probably stored near one another
in the file system. Hopefully this will get the user a bit closer to the
location of the next file they want to add.

-- 
   "Cut your own wood and it will warm you twice"