Resolve "Refactor debug messages from System.out to SLF4J"

- added "--logTarget" config parameter to Compiler, with console being default
- made handlers to templates so that inside variable content is possible
- refactored every System.out and System.err message to variable method
- fixed Errors test case after logging changes
- updated preprocessor-testing to not require handler-jadds as input anymore
- grammar: removed obsolete DefinitionFileName from Handler
- Compiler: revert to standard-out/-err instead of JUL for logging (slf4j did not work here), add some documentation snippets
- use slf4j loggers in tests instead of log4j ones
- always use slf4j as logTarget in build.gradle of tests
- update log4j impl to 2.17.1 in tests
- updated docu