Preserve files on exit (aka hot exit)

[Tyriar]

Opening this now to more easily track changes during development.

Fixes #101

TODO

• Backup files that are modified
• Add a delay to file backup to reduce amount of disk access
• Discard backups when files are no longer dirty
• Ensure no collisions occur with backup files
  • Untitled files
  • Text files
• Discard backups when a window is being closed and there is another window open
• Retain backups when the last window is being exited
  • Ensure backups are up to date
• Restore backups if available on first window launch
  • Restore text files
  • Restore untitled files
  • Launch a second window if the backup folder is different to that requested

Feedback Round 1

• Clarify "workspace" name in IBackupService (fixed in 1a29d14)
• Share BackupService between the main and renderer processes (fixed in c280e0c)
• Move BackupService to vs/platform/backup/node/ (fixed in 3387b6c)
• Opening empty workspaces currently errors out, disable completely for empty workspaces (fixed in 10aa709)
• Avoid sync calls where possible (fixed in 2b0a65a)

Clean up

• Resolve remaining TODOs
• Remove any console.log statements
• Add some tests
• Remove all empty lines at beginning of functions Preserve files on exit (aka hot exit) #12400 (review)
• Have backup service tests use a temporary folder for backups (needed to have tests pass on Travis)

Known issues/follow ups

• Hot Exit: Support multiple folder restore #13305 Support multiple folder restore
  • Behavior on Mac is a little strange right now due to the fact that the application is kept open when all windows are closed. This will not feel natural on Mac until restoring multiple workspaces is resolved (made notes there)
• Hot Exit: Support hot exit on empty workspaces #13733 Support hot exit on empty workspaces
• Hot Exit: Restore files after an editor crashes #12327 Hot Exit: Restore files after an editor crashes
• Fix dirty tab indicator after hot exit restore #14038 The tab dirty indicator is up to date right after restore, the file needs to be opened before it refreshes. See call to this.setDirty(true); in TextFileEditorModel.load
• Merge backup path logic from node/fileService.ts into backupService.ts #14037 Merge backup path logic from node/fileService.ts into backupService.ts

[Tyriar]

Merging this now as everything's checked off or has follow up tasks and the push/branch CI jobs are passing 🎉 (failing on master due to 9e1b35b)

The main thing to call out here is that restoring is disabled on Mac until #13305 gets done.