[FR] add persistent cache for offline use

[momobobe]

Pardon me for my poor network connection, but every time waiting for loading upon open makes me really annoyed. Please cache those data already downloaded for future use (even when offline).

[xarantolus]

I also have a really long loading time; I think that's because the Launch Library API isn't that fast, especially the first connection takes quite long.

The "first showing cached data and doing a refresh on launch" strategy makes a lot of sense in this situation, but I don't know when/if I'll get around to implementing it; but yeah I agree