(BroadcasterMixin apparently needs a one-off user-provided copy assignment op
due to SvtBroadcaster's odd design of having a user-provided copy ctor and an
explicitly deleted copy assignment op.)

Change-Id: I8fcb5a40f82031f94c37a65e9bd54657dfd07b7e
Reviewed-on: https://gerrit.libreoffice.org/56876
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>