`_type_` as context assist proposal for context schema

[tfesenko]

Extracted from https://github.com/RepreZen/KaiZen-OpenAPI-Editor/pull/402#pullrequestreview-67663598

I noted that there is a content assist proposal _key_ present in responses > content > schema. Not sure why it is there.

Test added in 0baa0ab

It happens because

1. We are combining all subtype contest assist proposals for a complex type, see com.reprezen.swagedit.core.assist.JsonProposalProvider.createComplextTypeProposals(ComplexTypeDefinition, AbstractNode, String):

       for (TypeDefinition definition : type.getComplexTypes()) {
           proposals.addAll(getProposals(definition, node, prefix));
       }

2. For each subtype, we call JsonProposalProvider.createObjectProposals(ObjectTypeDefinition, AbstractNode, String), which creates the _type_ proposal when there are no other proposals:

       if (proposals.isEmpty()) {
           proposals.add(new Proposal("_key_" + ":", "_key_", null, null));
       }

The given context is defined by a complex schemaOrReference type. reference is one of its subtypes, and it does not have other keyword-based context assist proposals, so we are adding the _key_ proposal.

Solutions

1. We can add the default _key_ proposal only when the aggregate result is empty.
2. Or, we can investigate if we need the _key_ at all and possibly remove it. _key_ was introduced before we had code-assist for the elements described with additionalProperties and was mostly added to cover the additionalProperties cases.