Improve performance in UI.

Other changes:
- Fix infinite scroll task cancellation & error handling.
When loading fails, infinite scroll list needs to check that the loader
is not changed before displaying errors.
- LoadPackageMetadata() should be moved out of the constructor of DetailControlModel.
Otherwise, it blocks the UI. Extract it into LoadPackageMetadataAsync().
- Add AutoDetectSourceRepository.
- DownloadCount and PublishedDate are displayed using the current culture format.
- Make getting package metadata calls parallel.