Consolidate adding add-on directories

This builds off of hrev46243 adding add-on directories all in one place
in AddOnMonitorHandler instead of repeating the code 3 times in
IndexServer, AddOnManager, and MediaAddOnServer.

The safe mode checking in InputServer is now redundant since it all
gets funneled into AddOnMonitorHandler::AddAddOnDirectories()
and the safe mode flags are checked there.

We should probably remove the InputServer::SafeMode() method, but,
I didn't want to break anything that depended on it so I left it.

headers/private/storage/AddOnMonitorHandler.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2004-2010, Haiku, Inc. All rights reserved.
+ * Copyright 2004-2013, Haiku, Inc. All rights reserved.
  * Distributed under the terms of the MIT License.
  */
 #ifndef _ADD_ON_MONITOR_HANDLER_H
@@ -45,6 +45,8 @@ public:
 	virtual	status_t			AddDirectory(const node_ref* nref,
 									bool sync = false);
 
+			status_t			AddAddOnDirectories(const char* leafPath = "");
+
 protected:
 	// hooks for sub-class
 	virtual	void				AddOnCreated(