Add command function for shell surface to set position

Targets #419.

This is my (WIP) PR for a command for setting the position of a
(shell) surface.

The actual implementation of the command _should_ be ready, but I
did not yet add code for setting the position of a shell surface
by now, as I think I need help from @TheNeikos for this.

Please do a short review if things look right by now
(especially @TheNeikos).
Nope, he checks whether the first two arguments are sane here.