Studio Possible adjust sensor+arrow or just arrow, but not just sensor

[KatieWoe]

For phetsims/qa#508. Win 10 Chrome
When looking at the sensor nodes, you can do things like adjust the opacity. However, it seems like you can either do this with the sensor and the arrow, or with just the arrow, but you can't do it with just the sensor. This seems a bit odd. If it is intended, feel free to close.

[zepumph]

Unassigning until phet-io redesign in #169