Generator: Reimplement internal MWE2-based workflow in Xtend

[tjuerge]

Quotes from forum:

Using MWE2 for Sculptor generators internal workflow has no advantage (the workflow isn't exposed to the user) but adds only complexity (e.g fiddling with classloaders).

If we have to have XTend integrated into every entity project because of templates overrides, we can use high level 'Workflow' class written in XTend which can be used also by users to customize. Initialization of MWE2 is taking about half second. It's nice to have some script at top which control generation but for us it should be XTend or Java class.