Rewrite `SingletonStage` for safety & power

Also, add more documentation so this is actually a usable piece of code.

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

@@ -1,39 +1,75 @@
 package org.gerweck.scalafx.util
 
+import scalafx.beans.property.{ ObjectProperty, ReadOnlyObjectProperty }
 import scalafx.stage.Stage
 
 import org.log4s._
 
-/** A stage that should only be open at most once per application. */
+/** A stage that should only be open at most once per application.
+  *
+  * To use this, you should do something like this:
+  * {{{
+  * class AboutWindow extends SingletonStage {
+  *   type InstanceStage = AboutStage
+  *   protected[this] def makeStage() = new Stage with AboutStage
+  *   trait AboutStage extends super.ParentStage { stage =>
+  *     title = "About My Application"
+  *     scene = new Scene {
+  *       ???
+  *     }
+  *   }
+  * }
+  * }}}
+  */
 abstract class SingletonStage {
   private[this] val logger = getLogger
 
+  /** The specific type of the underlying stage that you'll create. */
   type InstanceStage <: ParentStage
 
-  protected[this] final var singletonStage: Option[InstanceStage] = None
+  private[this] object State {
+    private[this] val stageProp = ObjectProperty(Option.empty[InstanceStage])
+    def currentStage = stageProp.value
+    def currentStage_=(stage: Option[InstanceStage]): Unit = {
+      stageProp.value = stage
+    }
+    def stage = stageProp.readOnly
+  }
+
+  /** Get the current stage (or `None` if it doesn't exist). */
+  def stage: ReadOnlyObjectProperty[Option[InstanceStage]] = State.stage
 
   protected[this] def singletonStageName = getClass.getSimpleName
 
-  protected[this] def makeStage(): Stage with InstanceStage
+  protected[this] def makeStage(): InstanceStage
 
+  /** Create the stage, or give it focus if it already exists.
+    *
+    * This needs to be executed within the UI thread. If you're not already within some kind
+    * of event handler, you need to use [[scalafx.application.Platform.runLater]].
+    */
   def showStage(): Unit = {
-    singletonStage match {
+    State.currentStage match {
       case Some(stg) =>
         logger.debug("Singleton ${name} stage is already open")
         stg.requestFocus()
       case None =>
         val ns = makeStage()
-        singletonStage = Some(ns)
+        State.currentStage = Some(ns)
         ns.show()
     }
   }
 
   protected[this] trait ParentStage extends Stage {
-    require(singletonStage.isEmpty, s"Cannot have two ${singletonStageName} stages")
+    require(State.currentStage.isEmpty, s"Cannot have two ${singletonStageName} stages")
     logger.trace(s"Creating singleton ${singletonStageName} stage")
 
-    override def close() = {
-      singletonStage = None
+    /** Override this if you need to provide custom `close` behavior. */
+    protected[this] def onClose(): Unit = ()
+
+    override final def close() = {
+      State.currentStage = None
+      onClose()
       super.close()
     }
   }