Also change behavior for the GetListenerCount() method which now
returns the count of listeners.

The previous behavior is available in method GetSizeOfVector().

Change-Id: I5b03fa55a309f4ff5aea5e8830c137786fc07e89
Reviewed-on: https://gerrit.libreoffice.org/10344Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>