BMailAccountSettings: use BPathFinder.

* This allows to put add-ons in non-packaged folders, too.
* Also, Set{In|Out}boundAddOn() only ever looked in the system dir.

headers/os/mail/MailSettings.h

@@ -171,6 +171,8 @@ public:
 
 private:
 			status_t			_CreateAccountFilePath();
+			status_t			_GetAddOnRef(const char* subPath,
+									const char* name, entry_ref& ref);
 
 private:
 			status_t			fStatus;