Simplify DialogButtons behavior

The custom region doesn't quite work yet, so just expose the underling
bar so we can still get the shared behavior. Maybe the region will get
fixed later on.

src/main/scala/org/gerweck/scalafx/util/control/DialogButtons.scala

@@ -7,6 +7,7 @@ import scalafx.Includes._
 import scalafx.event.ActionEvent
 import scalafx.event.ActionEvent
 import scalafx.geometry.Insets
+import scalafx.scene.Node
 import scalafx.scene.control._
 import scalafx.scene.layout._
 import scalafx.stage.Stage
@@ -71,17 +72,25 @@ class DialogButtons(stage: Stage, showOk: Boolean = true, showApply: Boolean = t
     }
   }
 
-  protected[this] def mainLayout = new VBox(Separators.horizontal, bar) {
+  protected[control] def mainLayout = new VBox(Separators.horizontal, bar) {
     hgrow = Priority.Always
   }
 
   delegate.asInstanceOf[DialogButtons.DialogButtonsJFXRegion].addChild(mainLayout)
 }
 
-private object DialogButtons {
-  class DialogButtonsJFXRegion extends JRegion {
+object DialogButtons {
+  private class DialogButtonsJFXRegion extends JRegion {
     protected[control] def addChild(n: JNode) = {
       getChildren.add(n)
     }
   }
+
+  def withCallback(stage: Stage, showOk: Boolean = true, showApply: Boolean = true, showCancel: Boolean = true)(applyChanges: () => Unit): Node = {
+    val ac = applyChanges
+    val db = new DialogButtons(stage, showOk, showApply, showCancel) {
+      override def applyChanges() = ac()
+    }
+    db.mainLayout
+  }
 }