No cursor change on hover

[zepumph]

I noticed for for #154, during my first time using this sim, that it seems a bit strange to me that the "pointer" cursor only appears once you begin dragging the elements in the sim. I would have expected a cursor change on hover to help cue what is draggable. For example, it wasn't immediately clear to me that only the left pencil was draggable, and not the right one (the refracted image).

[kathy-phet]

Fix this for prototype, if easy.

[pixelzoom]

I was asked to evaluate this issue for https://github.com/phetsims/geometric-optics/milestone/1. Fixed in the above commit.

@arouinfar please review in master. All draggable objects now have cursor: 'pointer' (the hand) when you hover over them. The exceptions are the lens and mirror. Since they can only be moved up/down, they have cursor: 'ns-resize' (up/down arrows).

[pixelzoom]

9/23/21 design meeting: Looks good, closing.