Support clearing on links

Because links register themselves into the app's click listeners, it's important that we clean them up when a link element is removed.

Support was already there for passing on a `clear` message to the DSL's `@gui` instance variable--we just didn't have it implemented right in `Shoes::Swt::Link`.

`replace` was also calling into the same clearing routines which weren't working, so this change also got that up to snuff.

This fixes #631 and #645, and gets rid of one of our few remaining pending specs.