fix group number-background-color state in group panel when entries s…

…elected

the cause of this bug is that the local variable "pseudoClassState" is garbaged by java gc.
there is no reference to this variable except an weak one from "bind" function.

src/main/java/org/jabref/gui/util/BindingsHelper.java

@@ -20,6 +20,7 @@
 import javafx.collections.ObservableMap;
 import javafx.css.PseudoClass;
 import javafx.scene.Node;
+import org.fxmisc.easybind.EasyBind;
 
 
 /**
@@ -41,23 +42,7 @@ public static <T> BooleanBinding all(ObservableList<T> source, Predicate<T> pred
     }
 
     public static void includePseudoClassWhen(Node node, PseudoClass pseudoClass, ObservableValue<? extends Boolean> condition) {
-        BooleanProperty pseudoClassState = new BooleanPropertyBase(false) {
-            @Override
-            protected void invalidated() {
-                node.pseudoClassStateChanged(pseudoClass, get());
-            }
-
-            @Override
-            public Object getBean() {
-                return node;
-            }
-
-            @Override
-            public String getName() {
-                return pseudoClass.getPseudoClassName();
-            }
-        };
-        pseudoClassState.bind(condition);
+        condition.addListener((obs, oldValue, newValue) -> node.pseudoClassStateChanged(pseudoClass,newValue));
     }
 
     /**