Cleanup for link click registration weirdness

*This is what I had in mind when I logged #639. Looks like Links are the only
weird type floating around.*

For whatever reason when I last revised the Link class it ended up with this
weird Proc wrapper being added at the SWT layer to make sure links got updated
properly.

It's much simpler to just make sure the DSL passes along changes to the block
we want called on click.

Because of a bit of special link behavior this doesn't actually use the
Shoes::Common::Clickable module, but it does leverage
Shoes::Swt::Common::Clickable on the backend, and is closer to the standard
click implementation.

cc/ @KCErb @PragTob