Voltmeter probes read over invisible objects in instance proxies

[BryceAG]

In instance proxies objects can be made invisible, which generally means non-interactive. However, the voltmeter will still read over objects as though they were still there. The image below shows this with the capacitor set to invisible:

(ignore the black background and white text nodes on the right, as that is a computer specific issue)

It is not important for this to be fixed for phetsims/phet-io-wrappers#49 but should be revisited at the next phet-io design meeting.

[BryceAG]

additionally the voltmeter can be put away even if the toolbox is not visible

[jonathanolson]

If we're hiding view elements, that would presumably not affect the model (so the voltmeter reading objects makes sense). We'd want to provide more explicit support for this type of thing.