DatabasePlugin: add interface DatabaseListener

Allow database plugins to announce that they have been modified.

src/Instance.hxx

@@ -21,11 +21,13 @@
 #define MPD_INSTANCE_HXX
 
 #include "check.h"
+#include "DatabaseListener.hxx"
+#include "Compiler.h"
 
 class ClientList;
 struct Partition;
 
-struct Instance {
+struct Instance final : public DatabaseListener {
 	ClientList *client_list;
 
 	Partition *partition;
@@ -48,6 +50,9 @@ struct Instance {
 	 * Synchronize the player with the play queue.
 	 */
 	void SyncWithPlayer();
+
+private:
+	virtual void OnDatabaseModified();
 };
 
 #endif