Keep content assist popup open even if user input has no matches

[trancexpress]

Eclipse bugzilla ticket: https://bugs.eclipse.org/bugs/show_bug.cgi?id=545075

As the title goes, ideally keep the content assist pop-up as the user types (aside from a few special characters such as empty space). By doing this, the user doesn't have to invoke content assist again after some typo.

I'm guessing this is partially work that would be needed for: https://github.com/eclipse-platform/eclipse.platform.ui/issues/888