allow navigation to event listeners and publishers in Eclipse environments

At the moment, the support for navigate from event listeners to event publishers and vice versa is implemented on the language server side and can be accessed in VSCode via the Find References menu item.

Unfortunately, this isn't really nicely accessible in the Java editor in Eclipse environments. Opening Java source code with the generic editor in Eclipse shows that the feature works in general, but that is really far from usable when working with Java source code.

One idea would be to use gutter icons for this kind of navigation, maybe there are other ways to do this, too. We need to look into the best way to do this integrated into the regular Java editing experience. We need to take into account that there might be several references that users would like to jump to, too.