fdo#43637 Two commands with the same name "Backspace"

One of them emulates the "Backspace" and the other one "Shift+Backspace", so I renamed it accordingly

Change-Id: I8a2a9f2b5b7212d4ea385c3b5f2e47ac014accc3
Reviewed-on: https://gerrit.libreoffice.org/6887
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>