fdo#76754 Speed up registration of new listeners to SfxBroadcaster

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/10344
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>