Better approach for fdo#40077: Characters cropped in GTK-Combobox

Now using gtk_widget_size_request() to get the native size.
I reverted the earlier change (border_width etc. ignored)

Change-Id: I0d89a2df5186bd27cbb77c8b0f2cc7b1fb247005
Reviewed-on: https://gerrit.libreoffice.org/6478
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>