Improve performance in filtering/sorting Lexical References

* Make LexSense.MinimalLexReferences use back references
* Refactor/repair fix to MinimalLexReferences
* Use LINQ in new LexReference service
* Performance refactoring in LexSense.FullReferenceName
* Refactored LexReference.FullDisplayText (with JohnT)
* Fix bold bug due to refactoring
* Add some comments

Change-Id: I1641df5f8a1bd62625f78b0dd7239191c0f106d1