Support toggle on Shoes::Swt::Background
Use @real when toggling

Don't need to go all the way back to the @dsl.gui.real object. We're already
in the context of the @dsl.gui.
Spec toggle for Shoes::Swt::Image

Modified Shoes::Swt::Common::Toggle to allow @real to not support set_visible
as the SWT element for images don't have it. Toggling was already provided on
that class, though, in the underlying paint listener.