New class for making singleton windows

This ensures there will be at most one copy of the stage, which will be
created and destroyed as needed. Trying to create a second just brings
the first into focus. This is good for things like preferences windows.

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

@@ -0,0 +1,35 @@
+package org.gerweck.scalafx.util
+
+import scalafx.stage.Stage
+
+import org.log4s._
+
+/** A stage that should only be open at most once per application. */
+abstract class SingletonStage {
+  private[this] val logger = getLogger
+
+  type InstanceStage <: ParentStage
+
+  protected[this] var singletonStage: Option[InstanceStage] = None
+
+  def name = getClass.getSimpleName
+
+  protected[this] def makeStage(): InstanceStage
+
+  def showStage(): Unit = {
+    singletonStage match {
+      case Some(stg) =>
+        logger.debug("Singleton ${name} stage is already open")
+        stg.requestFocus()
+      case None =>
+        val ns = makeStage()
+        singletonStage = Some(ns)
+        ns.show()
+    }
+  }
+
+  protected trait ParentStage extends Stage {
+    require(singletonStage.isEmpty, s"Cannot have two ${name} stages")
+    logger.debug(s"Creating singleton ${name} stage")
+  }
+}