db_plugin: introducing a plugin API for the song database

First draft, not really pluggable currently - hard-coded to use the
"simple" plugin, and calls several of its internal functions.

The API is very simple currently, all searches are still performed
over the root "directory" object.  Future changes to the API will move
those search implementations into the plugin, to allow more efficient
implementations, or implementations that don't have the whole tree in
memory all the time.

src/database.h

@@ -25,6 +25,7 @@
 #include <sys/time.h>
 #include <stdbool.h>
 
+struct config_param;
 struct directory;
 
 /**
@@ -32,18 +33,12 @@ struct directory;
  *
  * @param path the absolute path of the database file
  */
-void
-db_init(const char *path);
+bool
+db_init(const struct config_param *path, GError **error_r);
 
 void
 db_finish(void);
 
-/**
- * Clear the database.
- */
-void
-db_clear(void);
-
 /**
  * Returns the root directory object.  Returns NULL if there is no
  * configured music directory.