API simplifications for extensions

- Marker abstract classes for themes, features and plugins have been
removed. Now only AbstractExtension is used.
- Removed getVersion() method from the Extension interface. It was
useless.
- All existing extensions have been updated accordingly
- Linked with dandelion/issues#108
- Renamed ResourceHelper to FileUtils for more consistency
- Fixed typos in Javadoc
Removed ugly sysout