Use TextBuffer in VirtualProjectProvider so that it works for dirty files

Fix #452.
Merge pull request #468 from dungpa/overhaul-language-service

Use TextBuffer in VirtualProjectProvider so that it works for dirty file...
Cache project options properly

Orphaned projects are only checked in Debug build
Remove unnecessary use of text search service

It may help reduce CPU pressure on large files
Merge pull request #469 from dungpa/optimize-highlight-references

Optimize highlight references
update FCS to 0.0.50

uncomment LanguageService test checking find usages of properties with explicit getters / setters
Fix getting symbol usage scope

Close #471.
Merge pull request #472 from vasily-kirichenko/FCS-0.0.50

update FCS to 0.0.50
Add IsForStandaloneScript property
Merge pull request #473 from dungpa/standalone-scripts

Fix getting symbol usage scope