Xml.getAttributeOption.
Introduced Displayers - separate from Structure.

--HG--
rename : viewer/src/main/scala/org/podval/judaica/structure/Element.scala => viewer/src/main/scala/org/podval/judaica/structure/ElementDisplayer.scala
rename : viewer/src/main/scala/org/podval/judaica/structure/NonDivElement.scala => viewer/src/main/scala/org/podval/judaica/structure/NonDivElementDisplayer.scala
rename : viewer/src/main/scala/org/podval/judaica/structure/TanachStructure.scala => viewer/src/main/scala/org/podval/judaica/structure/TanachDisplayers.scala