Part 2: Rewrite of rest entry-point and gin generators

So the main branch has a lot of changes and I decided to split it up in multiple branches. The gist of this update is basically a near-complete rewrite of the Rest-Dispatch generators to allow extensions. Because of some type-safety concerns about Resource Delegates, this feature has been removed from the main repo. and will be added as an extension.

Brace yourselves as there are 14 branches so far, and I'm still not 100% done with the changes!

Also when I'm done rewriting the resource delegate generators, I'll open a new set of PR in https://github.com/ArcBees/gwtp-extensions
Halts the process. The return is sugar for the compiler instead of writing `return null` after.