vimode: add another motion command for moving down one line

I played around with vim recently and discovered some cursor movement command by accident... so of course I tried it in Kate and noticed it didn't work there. Now it does ;-)

While I was at it I also added some extra test for the - command to ensure it will always be aware of non-blank characters.

See also: https://vimhelp.org/motion.txt.html#%2B