Delay the enumeration of files not showing in the solution explorer.
Delay the enumeration of files not showing in the solution explorer.

This improves Project load time in the most common scenario.
Tested with Ember sample project, reduced from 13,000 to 1,700

Microsoft/nodejstools,lukedgr/nodejstools,Microsoft/nodejstools,kant2002/nodejstools,kant2002/nodejstools,lukedgr/nodejstools,lukedgr/nodejstools,lukedgr/nodejstools,Microsoft/nodejstools,paulvanbrenk/nodejstools,Microsoft/nodejstools,kant2002/nodejstools,paulvanbrenk/nodejstools,paulvanbrenk/nodejstools,paulvanbrenk/nodejstools,kant2002/nodejstools,lukedgr/nodejstools,paulvanbrenk/nodejstools,kant2002/nodejstools,Microsoft/nodejstools