* Introduce a editing frame with a button for drop-down form field.
** The frame is mouse transparent.
** Pushing the button opens the popup window with the items of the field.
* The button is visible when the cursor is inside the field.

Change-Id: I5c7db138d14380899fee046c95a5afe14cfea213
Reviewed-on: https://gerrit.libreoffice.org/68961
Tested-by: Jenkins
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>