* Added Lock()/Unlock() for explicit locking by a service user.
* Added NotifyLocked() and made Notify() inline.
* Added HasListeners() so one can check whether there is a listener at all
before preparing the event message.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@34736 a95241bf-73f2-0310-859d-f6bbb57e9c96