Resolves: fdo#73936 make FormFieldDropDowns a real fieldportion

split the checkbox and list and use a field portion for the list.
That way it knows how to line break correctly wrt hardspaces and doesn't hang
when the situation arises.

(cherry picked from commit a52ee51269a47e52d68405caf8507e1abaa6fd8f)

Conflicts:
	sw/source/core/text/itrform2.cxx
	sw/source/core/text/portxt.cxx
	sw/source/core/text/portxt.hxx

Change-Id: I46d73f19ef8e51e7c21c8783ce70bb80d98a784c
Reviewed-on: https://gerrit.libreoffice.org/8776
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>