[JENKINS-21216] - Added support for Folder job type:
* Allow type attribute 'Folder' in generated job
* Fixed updating generated folder job
* Allow using folder template
* Removing generated folder jobs
* Properly handle attempt to queue folder job
* Allow using templates inside fold	ers
Introduced hierarchical folder DSL methods, see suggestion
https://github.com/jenkinsci/job-dsl-plugin/pull/120#issuecomment-31766493