Remove use of Xtend templates so we do not depend on xtend-maven-plugin

Xtend used to have an edge of Java back when we started – it supported a number of features Java (7/8) lacked.

Today we are at a point where we have Java 11 at our disposal, which is superior in almost every department except for multiline text blocks. This advantage is disappearing with Java 15 support for Text Blocks.

We have a grand total of 4654 lines in .xtend files, hence we should be able to replace them rather easily, provided we create a few utilities to essentially give us a reasonable DSL.