Allowed RenderManShaders to have connections from empty ShaderSwitches.

Sometimes it's easier to wire up a switch before you wire up its inputs.
Improved Node::plugSetSignal() behaviour.

The signal is now emitted for all the outputs of the plug being set - otherwise we couldn't use plugSetSignal() effectively for plugs which had been promoted to Box level. With this new change, the Switch node can now have its index plug promoted, as demonstrated by the two new test cases.