Clarified interface of ChangedResourcesCollector. Do not accept non-Java files any more.
Job is only scheduled when there are actually changes.
When an editor is selected and the corresponding Class View node is visible, that node gets highlighted and comes to the front.
When an editor is selected and the corresponding Package View node is visible, that node gets highlighted and comes to the front.
First attempt to add toggle buttons to the UI. Does not work yet...