Fix calculated folder size to use unencrypted size

The encrypted size was used when calculating folder sizes.
This fix now also sums up the unencrypted size and shows that one when
available.

Fixes #6584

Please review @schiesbn @DeepDiver1975 @karlitschek @icewind1991 

Note that disabling or enabling encryption doesn't recalc folder sizes, probably something for another PR.

I'd like to wait for @schiesbn 's review before merging.