In the current unity implementation when an indicator is opened, there's no way to send it scroll or middle-click events.
This commit fixes this, applying an exception to the case of the appmenu entries, that should handle the middle-click like any other click.
Testing this with autopilot plus a fake indicator which reports events should be doable in the near future.. Fixes: https://bugs.launchpad.net/bugs/722082. Appoved by Tim Penhey.