Draft: An ugly fix prototype for downloading molecules with KNS.

As documented in bugs https://bugs.kde.org/show_bug.cgi?id=461027 and https://bugs.kde.org/show_bug.cgi?id=457443, downloading new molecules with the integrated KNewStuff is somewhat broken at the moment. The code used is deprecated, and the problems are likely caused by the new backend not handling the old-format(?) ProvidersUrl=https://data.kstuff.org/cgi-bin/hotstuff-provider?site=kalzium properly, giving filename as hotstuff-provider only, instead of the proper content filename. (My guesstimates, only lightly based on inspected facts so far)

Proper fix would probably be migrating the Kalzium KNS data to the new data source format, or possibly fixing support for the old data format. This commit makes the functionality kind of work, however.

The original behaviour in MoleculeDialog::slotDownloadNewStuff is somewhat peculiar, as the downloaded molecules are just temporarily stored under user app data, and moved to ~/Documents right after closing the dialog, which, I believe, breaks uninstalling downloaded content in any case.

I might inspect this more later.