Notification: Allow dbus-activating applications that are not running

This commit removes an isValid() check that will always return false when the target application is not running, even though that application may exist, be activatable, and implement the required interface while running.

By removing this check, we allow dbus to activate the application, which in turn will appropriately deliver the dbus messages generated by interacting with the notification.

I didn't see a safe alternative way to test whether the target application would implement the appropriate interface when running, but if anyone knows one I'd be happy to add it.