switch project performance improvements

[redmunds]

Changes:

• Do not check disk for file existence when opening files for a project, just add them to the working set. Handle case where a working set file no longer exists.
• Fix ChangedDocumentTracker so it doesn't read each file from disk as they are added to working set. Start/stop tracking documents when created/deleted.
• Handle adding/removing multiple working set files at a time to minimize how many times re-layout is done for working set and project tree once (i.e. not for each file). Note that re-layout is triggered when calculating whether to draw shadow, selection rectangle, and selection triangle.
• Also implement these improvements for multiple file open

[gruehle]

Is it possible for a document to remain loaded, even after its project has been closed? If so, this check will fail and the listener won't be removed. It should be safe to just unconditionally remove the listener.

[redmunds]

I can't think of how that could happen, but I made the change to be safe.

I also fixed a few other JSLint warnings in this file.

[gruehle]

Initial review complete.

[redmunds]

Finished with updates for initial code review.

[gruehle]

Looks good, and definitely speeds up project loading! Merging.