Various code changes (should have been split in multiple commits)

* Rename "Add/RemoveFile" to "Register/UnregisterFile"
* No-op implementation of GetDirectoryStatisticsRequest
* re-organize FileSystemTree tests.
* No-op implementation of "GotoNext/PreviousLocation" command handlers.
* Hook up "IVsRegisterPriorityCommand
* Expose a "DisposeContainer" on IVisualStudioPackage to allow adding custom lambdas to be called when the VS package is disposed.
Optimize using directives.
Optimize using directives.
Optimize using directives.
Optimize using directives.
Merge branch 'master' of https://github.com/chromium/vs-chromium