Start adding standard `StringConverter`s

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

@@ -0,0 +1,14 @@
+package org.gerweck.scalafx.util
+
+import scalafx.util.StringConverter
+
+/** A collection of premade string converters.
+  *
+  * @author Sarah Gerweck <sarah@atscale.com>
+  */
+object StringConverters {
+  /** A converter for an `Option[String]` that treats empty strings (or those
+    * with only whitespace) as `None`.
+    */
+  lazy val OptionStringConverter = StringConverter[Option[String]](s => if (s.trim.isEmpty) None else Some(s), _.getOrElse(""))
+}