Navigate by Control only works in design mode so turn design mode on
when Navigate by Previous or Next Control button is pressed.

Change-Id: I26a565758672c8dd1ec8bf979b199e8b240424e3
Reviewed-on: https://gerrit.libreoffice.org/70644
Tested-by: Jenkins
Reviewed-by: Jim Raykowski <raykowj@gmail.com>