Change plugin's content assist contributors to support asynchronous non-blocking completions

[difflaender]

Currently, installing the GWT Eclipse plugin disables non-blocking proposal computation in Eclipse because (according to the UI message) com.gwtplugins.gwt.eclipse.core.jsniCompletionProposalComputer does currently not set the flag that was introduced in Eclipse 4.14:
https://eclipse.dev/eclipse/news/4.14/jdt.php#completion-ext-flag-uithread

It's possible that further adjustments with regards to thread safety could be needed in code.

[difflaender]

For reference, these are the changes that the Groovy Eclipse plugin had to do to make its own proposal computations async:
groovy/groovy-eclipse@de5aced
groovy/groovy-eclipse@da8a60c

[protoism]

Hi @difflaender, this ticket might be super simple, or a nightmare, depending on thread safety issues which are not very simple to analyze.
I had a quick look at https://github.com/gwt-plugins/gwt-eclipse-plugin/blob/main/plugins/com.gwtplugins.gwt.eclipse.core/src/com/google/gwt/eclipse/core/editors/java/JsniMethodBodyCompletionProposalComputer.java and... things might be safe, it all depends on how Eclipse work under the hood.
Possibly the code to retrieve image looks "suspicious" from a threading point of view.
Why don't you try to have a look at it?
It should not be that complicate.