Let us solo a Layer, even if it is invisible

Let us solo a Layer, even if it is invisible - maybe with a kind of 'peep' mode in which the layer is shown while the solo button is pressed, and is made invisible again when the button is released.

Sometimes we need to see what a hidden layer (e.g. a matte) is looking like. Inthe current version, we need to turn on visibility first before it can be soloed. That is rather tideous.