Add new automatic conversion to `BooleanProperty`

There are a number of signatures in ScalaFX that won't accept an
observable of a Scala Boolean and that require you to convert it to a
Java Boolean. This adds an automatic conversion to resolve that.

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

@@ -0,0 +1,15 @@
+package org.gerweck.scalafx.util
+
+import scala.language.implicitConversions
+
+import scalafx.beans.property._
+
+trait LowPriorityImplicits {
+  implicit def scalaBooleanToBooleanProperty(ob: ReadOnlyObjectProperty[Boolean]): ReadOnlyBooleanProperty = {
+    val b = BooleanProperty(ob.value)
+    ob onChange { (_, oldV, newV) =>
+      b.value = newV
+    }
+    b
+  }
+}

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

@@ -3,6 +3,7 @@ package org.gerweck.scalafx
 import language.implicitConversions
 import language.existentials
 
+import scalafx.Includes._
 import scalafx.beans.property._
 import scalafx.beans.value._
 import scalafx.event.subscriptions.Subscription
@@ -19,7 +20,7 @@ import scalaz._
   *
   * @author Sarah Gerweck <sarah@atscale.com>
   */
-package object util extends ObservableImplicits {
+package object util extends ObservableImplicits with LowPriorityImplicits {
   type Observable[A] = ObservableValue[A, _]
   type SimpleProperty[A] = Property[A, _]