Update job-dsl-core to 1.28 and enable IDE support

To enable IDE support I had to make some changes:

* declare jobs folder as source folder in build.gradle
* renamed job scripts to be valid identifier names (IDE support does not work otherwise)
* moved job scripts to root package to avoid missing package name warning in IDEA (though it is still possible to use packages for job scripts)
* use DslFactory to enable IDE support in builder classes