Add KeyboardDragListeners to MeasuringTapeNode

[zepumph]

Over in phetsims/my-solar-system#86 it seemed worth a bit of exploration. Seems like the hardest part is just reusing the same code efficiently between dragListener and keyboardDragListener.

[zepumph]

I think it would be nice for @jessegreenberg to review this. The conversion was pretty straight forward. The worst part was that I know that the DragListener is using a bit of an old pattern for supporting dragging, and it wasn't very plug and play to convert it to use positionProperty + offset + transform + dragBoundsProperty settings. Thus, the options passed to DragListener and KeyboardDragListener aren't very aligned. Is there anything else you see?

[jessegreenberg]

Changes to MeasuringTapeNode and KeyboardDragListener look right to me, thanks!

If the intent for keyboard input is "smooth" dragging, can we use the velocity options of KeyboardDragListener? I regret the default value of moveOnHoldInterval option, it should be a large value for the "drag delta" behavior. I made the change above, keeping the shift drag for the tip slower than the base. Does that seem OK?

[zepumph]

I like it! Thanks so much.

[zepumph]

• Add focus highlight around the whole base

[zepumph]

On second thought, the design team finds this box acceptable, because the measuring tape image is already kinda boxy: