Release resources, if the tab is deselected

This allows the garbage collector to work properly and reduces the
minimum heap size

client/src/main/java/ctbrec/ui/tabs/ThumbCell.java

@@ -644,4 +644,8 @@ public class ThumbCell extends StackPane {
             return new int[2];
         }
     }
+
+    public void releaseResources() {
+        iv.setImage(null);
+    }
 }

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

@@ -872,6 +872,15 @@ public class ThumbOverviewTab extends Tab implements TabSelectionListener {
             updateService.cancel();
         }
         queue.clear();
+
+        for (Iterator<Node> iterator = grid.getChildren().iterator(); iterator.hasNext();) {
+            Node node = iterator.next();
+            if(node instanceof ThumbCell) {
+                ThumbCell thumbCell = (ThumbCell) node;
+                thumbCell.releaseResources();
+                iterator.remove();
+            }
+        }
     }
 
     void suspendUpdates(boolean suspend) {