Add ability to independently disable cannon height and angle adjustments

[arouinfar]

For #244

Related issues
phetsims/scenery#1041
phetsims/scenery#1116
phetsims/bending-light#397

The cannon can be adjusted in two ways:

• dragging the barrel to change the angle
• dragging the cannon base/pedestal to change the height

Clients should be able to independently disable these interactions (whether by pickableProperty or its successor).

[zepumph]

I actually though it was easier to instrument on the listeners themselves, since for the height one the listener was added to many Nodes. Having the flexibility to disable based on the Node and/or listener is really nice.

Please experiment with PhET-iO elements like these, and let me know what you think:

height: projectileMotion.introScreen.view.cannonNode.heightDragListener.enabledProperty

rotation: projectileMotion.introScreen.view.cannonNode.barrelTopDragListener.enabledProperty

[zepumph]

I also fixed the cursor, so that it can come from the input listener instead of each Node that the single listener is added to. This further simplifies the file. Now, disabled input listeners will show the default cursor.

[arouinfar]

The behavior is looking really nice. One (hopefully) minor request -- when cannonNode.heightDragListener.enabledProperty is false can the cueing arrows be hidden?

[zepumph]

It has been done.

[arouinfar]

Looks good, thanks!