DDSXTY12 — Specification document cleanup

1. Remove references to type signature in XType specification: The concept of type signature has been removed from the XTypes specification. See OMG Issue No: 16561. However there are still references to it in the latest spec.
List of references:
Figure 39 - Dynamic Type Support
Annex D: DDS Built-in Topic Data Types - Remove references to equivalent_type_name and base_type_name from PublicationBuiltinTopicData, SubscriptionBuiltinTopicData and TopicBuiltinTopicData

2. Replace the definition of the_type within TypeObject from TypeIdSeq to _TypeId:
In the final version of "OMG Issue No: 16097" the spec allows association of one type to an endpoint to simplify implementations.
However, the spec kept the member "the_type" as a sequence instead of going back to a single element.

3. Error in IDL definition of TypeObject: Definition of MapType, SequenceType, and StringType and in Annex B is marked as EXTENSIBLE but it should be MUTABLE.

4. The "type" member in SubscriptionBuiltinTopicData and PublicationBuiltinTopicData should have "DynamicType" type instead of "TypeObject"

Proposed Solution: Make the suggested changes.

Incorporated corrections

Document cleanup performed removing dangling elements and incorrect IDL structures.