Extract an abstract class from DocumentationHover
Extract some utility methods and add correct support for callables
Also take in account the JDT DebugContext to retrieve the current frame
Manage variables retyped with new names, as well as captured variables
First implementation of Ceylon debug hover :
- supports :
    - local variables,
    - fields of the current class,
    - captured variables,
    - top-level values
- still to be added :
    - fields or attributes of enclosing scopes