we want the combobox to be narrower than it wants to be.  To make that happen
we have only the option of shrinking the cell renderer of the combobox area.
And that is also used by the menu.

Setting a small value of e.g. 1 works to let the combobox not request more
width than that, and the combobox will correctly render within the wider size
it actually gets. But then the menu is a min width menu, which is undesirable,
we want the menu to be as wide as it can be.

So calculate what part of the combobox belongs to the cell area and set
the widest cell area we can within the overall combobox width, resulting
in the widest menu we can get.

Change-Id: Ie3e9960a320a70471ac21d2a88f32632cafa951f
Reviewed-on: https://gerrit.libreoffice.org/68167
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>