Fix some issues with non-deployable curved solar panels (NIV-3 & NIV-18)

4dfcac1 hides the GUI buttons for "Deploy Panels", "Retract Panels", and "Toggle Panels" if the part does not have `Deployable = true`.

859fb13 makes it so that even if someone tried to use Action Groups to deploy/retract/toggle a non-deployable part, there won't be a `NullReferenceException` (without this patch, `Deploy` and `Retract` could get called, and `deployStates` would be `null`).

(Side note: I wanted to see if there was a way to hide the actions in the VAB Action Group editor entirely, but interestingly, even Squad's own non-deployable solar panel shows deploy/retract/toggle actions. So I guess hiding the actions isn't important, as they don't do anything now anyway.)

23d4e7f fixes a small typo in the documentation for `Toggle`.