Make workspace capable of providing some Executor

Java CompletableFuture are often convenient. Recently the Display was well integrated with those by making `Display implements Executor`. Similarly, we should facilitate usage of future to deal with the workspace model; to replace WorkpaceJob and workspace.run().
I imagine the Workspace could provide an `.executor()` and another `.executor(ISchedulingRule)` method so one could write code of the form:

```java
CompletableFuture.supplyAsync(() -> computeAProjectDescriptionInWhateverThread()) // locks nothing by default
.thenAccept(projectDescription -> createProject(projectDescription), workspace.executor()) // locks the workspace, like workspace.run(), then free it
.thenAccept(project -> navigator.select(project), display); // locks the UI
```