Clear threadpool queue when tab is changed

Clear the threadpool queue when the user switches between tabs, so that
the resolution tags, which are visible get updated earlier. Before, the
tags on tab the user is coming from were updated first and after that
the tags of the currently visible tab.

src/main/java/ctbrec/ui/ThumbOverviewTab.java

@@ -565,6 +565,7 @@ public class ThumbOverviewTab extends Tab implements TabSelectionListener {
 
     @Override
     public void selected() {
+        queue.clear();
         if(updateService != null) {
             State s = updateService.getState();
             if (s != State.SCHEDULED && s != State.RUNNING) {