Fix `ObservablePref` Scala 2.11 compatibility

Scala 2.11 doesn't automatically lift functions into implementations
of single-method interfaces, so we have to be a bit more explicit when
creating the callback object.

src/main/scala/org/gerweck/scalafx/util/prefs/ObservablePref.scala

@@ -1,7 +1,7 @@
 package org.gerweck.scalafx.util
 package prefs
 
-import java.util.prefs.Preferences
+import java.util.prefs._
 
 import scalafx.application.Platform.runLater
 import scalafx.beans.property.ObjectProperty
@@ -10,7 +10,7 @@ import org.gerweck.scala.util.prefs._
 
 /* TODO: take an implicit that will deteremine whether this is an `ObjectProperty` or what */
 class ObservablePref[A] protected (path: String)(implicit handler: Pref.AccessHandler[A], prefs: Preferences)
-    extends Pref[A](path) {
+    extends Pref[A](path) { thisPref =>
 
   lazy val observe: ObjectProperty[A] = {
     val initialValue: A = this()
@@ -23,23 +23,31 @@ class ObservablePref[A] protected (path: String)(implicit handler: Pref.AccessHa
      * in a bidirectional bridge. */
 
     /* Preferences => Property bridge */
-    prefs.addPreferenceChangeListener { pce =>
-      if (pce.getKey == path) {
-        runLater {
-          val currentValue = this()
-          if (property.value != currentValue) {
-            property.value = currentValue
+
+    /* In Scala 2.12, the callback could just be bare inside `addPreferenceChangeListener`.
+     * However, it must be created explicitly since we cross-compile against Scala 2.11. */
+    val changeListener = new PreferenceChangeListener {
+      def preferenceChange(pce: PreferenceChangeEvent): Unit = {
+        if (pce.getKey == path) {
+          runLater {
+            val currentValue = thisPref()
+            if (property.value != currentValue) {
+              property.value = currentValue
+            }
           }
         }
       }
     }
+    prefs.addPreferenceChangeListener(changeListener)
+
     /* Property => Preferences bridge */
     property.onChange { (_, _, newV) =>
       if (newV != this()) {
         this() = newV
       }
-      ()
     }
+
+    /* Return the bridged property */
     property
   }
 }