Support incremental annotation processing plus tests

[stephanenicolas]

This PR adds support for incremental annotation processing as defined by gradle'incap project: https://docs.gradle.org/4.7/userguide/java_plugin.html#sec:incremental_annotation_processing

All 3 annotation processors have been enhanced to become incremental and they all are isolating.

The PR has 2 parts:

• add support for incremental annotation processing: add the meta-inf file and make sure the originating element of all generated files is passed to the filer properly.
• add tests: I didn't want to add integration tests (they would ensure that incremental compilation truly works) as they would require quite some news mecanics (gradle testing + spock). So I simply added unit tests to make sure that originating elements are passed properly to gradle.

If we are interested by integration tests, I believe this one is well done: https://github.com/tbroyer/gradle-incap-helper/commit/2209bde0761dd30b4d45a943a97e2686650d343b

[stephanenicolas]

The build actually passes on travis, some post build command is failing.

[dlemures]

Good to merge