DevGuide_RefactorXtend2Java

Some steps to consider when refactoring compiled Xtend code to Java again (see #1569):

1. Disable Automatic Build
2. Move Java sources from xtend-gen to src
3. Auto-Format the source code
4. Perform Source / Clean Up action. See here for the cleanup actions settings to apply.
5. Remove @SuppressWarnings("all")
6. Start manual source cleanup
7. Move constructor to appropriate place. Often they are much too late in the code
8. Remove super() calls in constructors
9. Look for unnecessary final modifiers, esp. in parameters
10. Remove references to active annotations. The resulting code should not import anything unnecessary from org.eclipse.xtext.xbase.lib.
11. hashCode: Use java.util.Objects#hash()
12. Replace Guava Objects#equal() by Java Objects#equals()
13. Look out for double parantheses ((. Sometimes expressions are unnnecessarily enclosed by parentheses.
14. Remove unnecessary this. qualifiers
15. Update copyright header upper year
16. Remove unused imports
17. Use method references where appropriate
18. When nested classes are used, access to them e.g. in casts are unnecessarily qualified by the enclosing type name.
19. Resolve usage of Exceptions#sneakyThrow()