Fix type inferencing, syntax highlighting and code navigation for trait property references

[eric-milles]

https://stackoverflow.com/questions/61713375/trait-methods-are-not-fully-visible-in-groovy-eclipse

Consider the following:

trait T {
  String foo
}

class C implements T {
  def m() {
    foo
  }
}

Reference to "foo" is shown as unknown (underlined).

Unlike #953 and #960, the trait is declared in a separate source unit. This issue falls in a gap between #960 and #704. Some trait fields and methods are not represented in the JDT model (GroovyCompilationUnitDeclaration, GroovyTypeDeclaration and SourceTypeBinding) since they are not compatible with Java 7 interfaces and to prevent warnings for unimplemented abstract methods. Thus they are not available in a JDTClassNode that is created for source reference.

To be available for type inferencing, etc. the JDTClassNode for a trait needs to be populated with the missing fields, methods and properties.

[eric-milles]