Add ability to drag left edge to decrease scheduling period

Sometimes when scheduling by hand a gap can appear to the left.

As the scheduling GUI works right now its only possible to change the period by dragging the right edge.

Maybe it should be possible to drag the left edge as well.

added Scheduling GUI label

added Enhancement label

Note that a difficulty in doing this is that operations can lie "over-the-edge".

I think a "better"/alternative way is to rotate the schedule to the left #247 and then drag the right hand edge.