Create common layer for UI-centric plugins #147

There is a growing pattern for UI-specific plugins like the InputWidget implementations. These are HandlerPlugins or WrapperPlugins that should only be chosen if their associated UI(s) matches the activated UI(s). Rather than have this logic specifically at the level of InputWidget and InputPanel, we should generalize things to a common UIPlugin, all of which share the "is my UI active?" logic.

We could then use it for cases like imagej-updater's UpdaterUI plugins, which right now activate blindly whenever they are on the classpath.