Storing song objects in the Songs slice and using pointers to the
underlying array slots breaks if the underlying array must be grown.
When the array is grown the objects are copied to the new array and
the pointers used in queue point to the old objects.
This causes that Playlist.applyTimeBonus may not correctly apply
to the most recent list of songs.

This is fixed by always using Song pointers.

The 'local' backend is able to serve the music from a path specified
by '-server-music-dir'.

Add command line options to select the server port as well as a list
of backends.