Dropdown combo-/listboxes should support accessible action togglePopup

At least VoiceOver needs it to be able to open the dropdown in a native
way (through AXShowMenu). The list still isn't accessible on mac but
that is an other issue.

Change-Id: I825b54536dfc9743b722eb55873bde3d5eef953d
Reviewed-on: https://gerrit.libreoffice.org/9833
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>