Add file watcher to auto-refresh game list on Archive tab

[herzbube]

When an .sgf file is uploaded into the app via iTunes, the file does not automatically show up in the list of games on the "Archive" tab. Several workarounds currently exist (e.g. lock/unlock the device), but they all involve sending the app to the background.

#240 suggests adding a refresh button, but this still requires the user to take manual action to see the added file(s). A better approach would be to have a file watcher that automatically refreshes the list of games whenever it detects a change in the app's document folder.

This issue replaces #240.

[herzbube]

Initial research indicates that GCD (Grand Central Dispatch) can be used to receive notifications about filesystem changes.

• https://stackoverflow.com/questions/21827786/how-to-monitor-file-change-on-ios-platform
• https://stackoverflow.com/questions/1386743/observe-a-file-or-folder-in-objective-c