BatchModeTest: show UI on EDT

[hinerm]

Closes #306

[hinerm]

So the problem is that in the test we call service.showUI on the main thread, which of course then deadlocks with the EDT since we're using graphical components off the EDT.

But I'm not sure that we don't want to make this dispatch to the EDT elsewhere in the UIService stack..?

[ctrueden]

Is it possible to fix this in scijava-ui-swing instead?

[ctrueden]

Hmm... it turns out the UserInterface interface has a requiresEDT boolean flag, which is present all the way into SciJava Common. That's maybe a violation of separation of concerns, but it's already there, so we can use it: the base showUI implementation, independent of any user interface, could use threadService.invoke(() -> ui.show()) or whatever if that flag returns true?

[hinerm]

Closing in favor of scijava/scijava-common#481