Complete semantic predicates feature

[rmitin]

Pull request #174 introduced grammar fragment extension to support semantic predicates. The work is, however, incomplete.

Support for coloring and content assist is missing.
Comments are used where annotations are possible.

1. Complete the implementation by adding the missing support for semantic coloring and content assist.
2. Extend the test coverage
3. Replace @SemanticPredicate(methodName) comment with @SemanticPredicate annotation and derive method name from the rule name.