Add offline support

[rinigus]

Based on "packages" PR branch.

On start, Chum refreshes the repo to display latest packages. That fails badly in offline mode. In contrast, Storeman handles it very nicely. @mentaljam - any tips?

[rinigus]

Looks like it is not going to be very trivial and I'd relegate it to nice-to-have features for now. Unless we start seeing users complaining about it.

In principle, it is not trivial to detect offline mode. What is suggested is to try to load some online file before operation and see if it fails. In this respect, we can let packages to get refreshed as it is now and fail accordingly.

Adding cache to NetworkManager does not help in offline case as we are hitting https://bugreports.qt.io/browse/QTBUG-52053 . So, to get offline support, we would have to implement:

• custom cache solution that would be possible to use on errors
• custom network query class, probably bundled with the cache mentioned above to request data from cache on failure or if cache is fresh enough
• ideally, custom image provider backed by cache to keep app icons

All in all, significant amount of work and I feel there are more important issues to consider before.