Implement more AsyncSwingWorkers into Jalview-JS

I've laid out an example of how we can make any multi-pass process synchronous or asynchronous in Java and/or JavaScript. See SequenceFetcher and https://github.com/BobHanson/java2script/blob/master/sources/net.sf.j2s.java.core/src/javajs/async/AsyncSwingWorker.java

It would be good to consider other places where this might be valuable.