Mirror SDB API usage and fix as many CorDebug issues as possible!

Immediate window auto-completion and evaluation works.
Only one more pad corruption remains with hovering over NRefactory analysis on interfaces.
Put most of our code addition to extension classes. Left only "NotImplementedException" implementations in specific places. This minimizes difference from Mdbg sources.
Fixed many "Value out of expected range" issues with properties which didn't have either a getter or a setter.

Evaluation is no longer prone to dying. Fixed lots of exceptions thrown by the backend implementation.