SpringBoot let GroovyTemplateProperties extend AbstractTemplateViewResolver.

To be able to easily set the exposeRequestAttributes (amongst others) the GroovyTemplateProperties now extends AbstractTemplateViewResolver. This is also in line with the other templating solutions like Freemarker and Velocity.

Fixes: #3365

Comment From: philwebb

Thanks Martin. I've probably asked before but can you confirm that you've signed the CLA.

Comment From: mdeinum

You did :) and as well I did sign the CLA... I did some polishing to align it more with the Velocity/Freemarker configuration and updated the documentation.