Minor fixups to type signatures

src/main/scala/org/gerweck/scalafx/util/SingletonStage.scala

@@ -14,7 +14,7 @@ abstract class SingletonStage {
 
   protected[this] def singletonStageName = getClass.getSimpleName
 
-  protected[this] def makeStage(): InstanceStage
+  protected[this] def makeStage(): Stage with InstanceStage
 
   def showStage(): Unit = {
     singletonStage match {

src/main/scala/org/gerweck/scalafx/util/layout/Anchored.scala

@@ -1,8 +1,9 @@
 package org.gerweck.scalafx.util.layout
 
 import scalafx.geometry.Insets
+import scalafx.scene.Node
 import scalafx.scene.control.MenuBar
-import scalafx.scene.layout._
+import scalafx.scene.layout.{ AnchorPane, Priority }
 
 /** A wrapper that binds its child pane to all four corners so it will always
   * grow as the stage is resized.
@@ -12,7 +13,7 @@ import scalafx.scene.layout._
   * @author Sarah Gerweck <sarah.a180@gmail.com>
   */
 object Anchored {
-  def apply(margins: Insets = Insets.Empty, menus: Option[MenuBar] = None)(body: => Pane) = {
+  def apply(margins: Insets = Insets.Empty, menus: Option[MenuBar] = None)(body: => Node) = {
     new AnchorPane { ap =>
       hgrow = Priority.Always
       vgrow = Priority.Always