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