Merge #13043: [qt] OptionsDialog: add prune setting

cbede7dbfde83d53ef38d257e9940af5f163b03c [qt] OptionsDialog: add prune setting (Sjors Provoost)

Pull request description:

  The default suggested value is 2 GB. Minimum is 1 GB (550 MB rounded up).

  When the user toggles this setting, a strong warning appears that undoing requires re-downloading the chain:

  <img width="478" alt="schermafbeelding 2018-05-15 om 12 35 24" src="https://user-images.githubusercontent.com/10217/40051858-7939cc20-583c-11e8-9120-327a75376732.png">

  Tooltip points out that actual disk usage can be higher. It's a bit vague on the "advanced features", because I'm assuming anyone who needs to use `-rescan` and `-txindex` will read the documentation, and a more detailed text would needlessly confuse everyone else.

  <img width="450" alt="schermafbeelding 2018-05-15 om 12 33 51" src="https://user-images.githubusercontent.com/10217/40051791-49d6156a-583c-11e8-97b9-7de6dfd8c481.png">

  The UI uses gigabytes for readability and easy of use. There is also no manual pruning UI (`prune=1`). The user will have to use `bitcoin.conf` for those things.

  Fixes #6461. When combined with #13029 the user, after pruning their node, can safely reset settings and/or use bitcoind without having to edit `bitcoin.conf`. However I don't think that's an essential prerequisite.

Tree-SHA512: e17aff276d7235fbd40796adb6431d430620788a753ee13bc064abd35d2edc4280a3d3cddc18e42b4e00edff13ed18fd4f2a966c6f0b43b689afd13673e0c4bf

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