Fix legacy opener

Primarily fixes #28 
@ctrueden @dscho - I am concerned about the changes to `LegacyUI` so that `show` runs on the same thread.

I am assuming there was some reason that the `AbstractUserInterface` implemented `show` using `queue` instead of `invoke`? But the result is that the `DisplayService` will not block when publishing `DisplayCreatedEvent`s, and in the legacy layer we want that to happen so that the `LegacyImageDisplayViewer` will register an `ImagePlus` for us.