Merge RichAnnotationModel into AnnotationUtils

The only functional change is that JavaConversions is replaced
by the more explicit JavaConverters.
Remove Editor, EditorUI and RegionHelper

Those files were unused
Refactorings in EditorUtils class

- Non UI logic of `applyChangesToFileWhileKeepingSelection` is moved to
  its own method
- The class EditorHelper is removed because its content overlaps with
  EditorUtils (and it was unused anyway)
- The package ui.editor.util is removed
WIP