Add setting to choose how to open office documents

[commandervoid]

Summary

Add setting to choose how office documents (docx, xlsx, pptx,
etc.) are opened: Seafile's internal webview (default), in an external app,
or to be asked each time. Also carved out cache checking functionality
to avoid code duplication and fixed up an old test.

Changes

User-facing

• New preference "Office documents" in settings with 3 options:
  "Open in Seafile", "Open in external app", "Ask each time"
• "Ask each time" shows a dialog letting users pick webview or external app
• Context menu "Open with" goes straight to the system chooser (consistent
  with user intent to pick an external app)

Testing

• Unit tests for Utils.getOfficeFileClickAction() covering all code paths
  (no OnlyOffice server support, non-office file, INTERNAL/EXTERNAL/ASK)
• Unit tests for FileCacheHelper.checkRemoteAndOpen() covering
  not cached, a stale cache, and an up-to-date cache.
• Fixed up the NaturalOrderComparator unit test by having it exercise
  StringNaturalOrderComparator.

[zhwanng]

Thank you for your PR, this is a very useful feature

[commandervoid]

One thing I didn't do yet is set the app picker to have the 'Always open' option when the EXTERNAL option is selected. I'm planning to do that in a followup PR (or you can just change that here if you'd like).

[zhwanng]

Since the app already includes the ability to open Office-type files, and adding this feature would require additional configuration options, this PR will be put on hold for now.

For now, can open documents using the local app by following these steps:
Document list -> Long-press to open the context menu -> Open with