This is quite similar to SwFrame::GetFrameId(), i.e. it assigns a
numeric identifier to each instance to help debugging, as those
identifiers are stable accross runs.

Change-Id: I9cc57e316435f0284a1d481a956a703be859d67e
Reviewed-on: https://gerrit.libreoffice.org/27669Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>