when editing is enabled also delete and backspace should be possible
this applies e.g. to input fields within protected sections

Change-Id: Iee5ef133971a3f2967e6c92e685a16afa003cfe8
Reviewed-on: https://gerrit.libreoffice.org/19399Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Oliver Specht <oliver.specht@cib.de>