Refactor Template package

[johanjanssens]

• Implement filters using a behaviour approach
• Implement helpers using a behaviour approach
• Move loading logic into render() method and remove loadString() and loadFile()
• Add generic template filter to allow mixing different template types in the same template.

Breaking changes:

1. TemplateInterface::loadString() and loadFile() methods have been removed and the arguments of the render() method have changed. To migrate remove calls to loadString() and loadFile() and add the content or url as the first argument to render() call.
2. A helper no longer has a getTemplate() method. The TemplateBehaviorHelperable::invokeHelper() method passes the template object as the second parameter to the helper method being invoked.
3. A filter no longer has a getTemplate() method. The TemplateBehaviorFilterable::filter() method passes the template object as the second parameter.
4. ktml:block elements are considered foreign elements accoording to the html5 standard. Foreign elements need to have self-closing start tags when used without content. To migrate add a self-closing tag to empty ktml:block elements.