Revisit the display for "shift" mode

[pixelzoom]

When you select the "shift" mode, the control panel looks like this:

The amount of empty space here has always bothered me a bit. An easy change would be:

But maybe there's a pedagogical reason for why the icon is the way that it currently is. @amanda-phet @veillette your opinion?

[pixelzoom]

If we do want to change this, here's the change for CurveManipulationDisplayNode.ts:

        else if ( mode === CurveManipulationMode.SHIFT ) {
-         curve.shift( xMax, yMax );
+         curve.shift( xMax, yMin );
        }

[veillette]

I think it was an oversight. Given that the slider disappears in SHIFT mode, it looks better at the yMin position.

[amanda-phet]

this looks better to me, to make this change.

[veillette]

Done in the commit above. Here is the new look! (Ignore the red outline, I was using the ?dev queryParameter)

Assigning to @pixelzoom to review. Feel free to close.