Added support for letting user mail add-ons override system ones.

* The path will now be relativized before storing it.
* On load, the add-on will be tried to load from the user, then common
  and finally system add-on directory.

headers/os/mail/MailSettings.h

@@ -92,6 +92,9 @@ public:
 
 	virtual	bool				HasBeenModified() const;
 
+private:
+			const char*			_RelativizePath(const BPath& path) const;
+
 private:
 			BMessage			fOriginalSettings;
 			entry_ref			fRef;