There should be an (under development) SPI for adding java.hints-like hints. As part of this, current java.hints module should be split into several modules, at least to one containing the hints infrastructure and second containing the implementation of the hints themselves.
This is separation of UI from java.hints module. Java Refactorings also moved to refactoring.java.
Maybe this task can be closed. Now we have SPI for hints and also separated ui and refactorings from hints module.