* It was weird anyway that a drop-down form field was represented
as an CheckboxFieldmark.
* It will be useful for later commits, to have a separate field type
for drop-down field.
* Needed to fix-up the API a bit because it was designed to specify
the field type after initialization. I solved it in a way to not break
the API behavior. Hopefully it's not very slow.

Reviewed-on: https://gerrit.libreoffice.org/68960
Tested-by: Jenkins
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>
(cherry picked from commit f66a83c9)

Change-Id: I3103e6b1c36289b27b62ab9ca7dfeebc14901c8a
Reviewed-on: https://gerrit.libreoffice.org/69194Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>