Add shared context for SWT specs

Also improve encapsulation by exposing methods on the App
object so we don't have to reach inside it.
Improving encapsulation: Add #remove_listener to Swt::App
Don't dispose @real if it's nil (as in Slot)
Tag specs that fail on OS X, but succeed elsewhere
Update specs for shared context

Also, eliminate temporary test-double shims for allowing
differently-named doubles in shared specs
Remove commented-out code
Use app accessor consistently
Clean up specs a bit