bb-imager-gui: Add support for remote subitems

- Allows splitting config into smaller subitems list. The CI/CD Template
  [0] can be used to generate os_list in CI.
- Using a very simple loading text right now.

[0]: https://openbeagle.org/ayush1325/ci-components



Signed-off-by: Ayush Singh <ayush@beagleboard.org>