Cache albums as we go along

[oparoz]

In Gallery+, we're not mapping the entire cloud in memory any more, which means that every folder change will trigger a request to the server to get a new list of files.

We could improve the situation by caching what we receive and check that cache before making a new request.

[oparoz]

Make sure store the configurations as well since we're scanning the whole parent tree every time we enter an album.

[oparoz]

I didn't think it would make that much of a difference, but apparently we can notice a 100ms difference in loading speed... but the change is more dramatic if you have hundreds of pictures in a folder...

@jospoortvliet @deMattin, since you have large collections, see if you notice the difference when using master (for 8.0).

[oparoz]

This is now also available on the dev branch for 8.1

[oparoz]

Or you could use ultimatedev which also contains all the latest PRs :)