fix for the display of bulk-aktivities in the acitivity-tab

The bug-fix for the activity-tab in 4.4.5 was just to exclude bulk-mails
from display. This is not sufficient for us.
So we went back to 4.4.4 and added there a functionality that displays:
* one target-contact, if only one exists,
* the number of target-contacts, if more then one exist.