Prepare for the plugin interface.

Keep only a reference.

Fixes bogus assertion failure.

Prepare to make this a new plugin API, for example to use a SMB share
for the music_directory.

Fixes assertion failure when update gets launched during MPD startup.

Replaces global variable "main_loop".

Decouples db/update/Remove.cpp from global variables.

Don't use the global variable "instance".

Don't use the global variable "main_thread".

Move from db/DatabaseGlue.cxx, eliminating global variable.

Database update was randomly broken due to these uninitialized
variables.

Fixes loading database songs from state file (regression by commit
29072797).

Don't use the global variables from the DatabaseSimple library.

Remove dependency on the DatabaseSimple library.

Reduce global variable usage, move to frontend code.

Reduce overhead.  LightSong can always point to the "real" allocated
string.

Playlist file name first, to be consistent with the other functions in
this library.