Added move cell functions. Made the beginning/end calls return point

[Nidish96]

I've added two functions for moving a cell either up or down a cell. I've also made the begging and end functions return (point) after moving the point. Previously these were returning only when the conditional was met, so was unreliable for use in other functions. I've added prefix support for some of the navigation functions (moving, forward/backward) by basically looping over the argument. I hope this is the best way to support this.