Synchronize custom sword module folder

[tobias-klein]

This fixes #1023.

• Add basic library functions for recursively synchronizing Dropbox folders.
• Extend node-sword-interface to support an additional (custom) module folder.
• Add UI:
  • input field for configuring custom sword module folder
  • checkbox for enabling this feature
  • button in options menu for syncing on demand
  • Disable button in options menu if sync option is not configured
  • dialog for showing sync progress
    • add progress bar
    • add a button in the dialog for starting the sync (instead of starting immediately)
    • add cancel button
    • add info about the fact that the process may take longer
    • add info which Dropbox folder is synced
    • show total file size that is being synced
• Implement IPC functions for sync, including a callback for progress feedback.
• Integrate sync functionality: Sync on demand (button)
• Perform basic validation of remote directory (check existence, check for mods.d and modules directories)
• Augment sword module paths with custom module folder
• Handle errors
• Update locales
• Test on macOS
• Test on Windows
• Test on Android