SLING-3649 make Thymeleaf scripting configurable and extensible

- out of the box support for legacy HTML5 through embedded NekoHTML
- runtime configurable TemplateModeHandlers for XML, VALIDXML, XHTML, VALIDXHTML, HTML5 and LEGACYHTML5
- MessageResolver backed by ResourceBundleProvider from org.apache.sling.i18n
- uses UTF-8 charset for reading templates (not configurable)

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1601661 13f79535-47bb-0310-9956-ffa450edef68