Add a language neutral object model for FindRefs

Merge dev15-preview-4 into master

Update deployment VSIX names and ids.

The display names are shown if you add the feed URL, so being able to
distinguish between them is nice.

(Note: I also saw that we had a duplicate ID, so I changed the one we don't
seem to publish anymore)

Update DSRP to 1.2.0-beta1-60723-01 and SRM to 1.4.1-beta-24322-03

Trim common path elements for C#/VB find references, just like we do for TypeScript