Merge bitcoin-core/gui#125: Enable changing the autoprune block space size in intro dialog

415fb2e1ab GUI/Intro: Move prune setting below explanation (Luke Dashjr)
2a84c6bcf6 GUI/Intro: Estimate max age of backups that can be restored with pruning (Luke Dashjr)
e2dcd957fa GUI/Intro: Rework UI flow to let the user set prune size in GBs (Luke Dashjr)
f2e5a6b54f GUI/Intro: Abstract GUI-to-option into Intro::getPrune (Luke Dashjr)
62932cc686 GUI/Intro: Return actual prune setting from showIfNeeded (Luke Dashjr)

Pull request description:

  ![Screenshot_20200911_095102](https://user-images.githubusercontent.com/1095675/92933661-0c4cea00-f436-11ea-9853-2456091ffab3.png)

  Moved from https://github.com/bitcoin/bitcoin/pull/18728

ACKs for top commit:
  ryanofsky:
    Code review ACK 415fb2e1ab. Changes since last review: mb/gib suffixes, constexpr QOverload expected_backup_days tweaks, new moveonly layout commit
  jarolrod:
    Tested ACK 415fb2e.
  Talkless:
    tACK 415fb2e1ab, tested on Debian Sid with Qt 5.15.2.
  hebasto:
    ACK 415fb2e1ab, my unresolved comments are not blockers, and they could be resolved in follow ups.

Tree-SHA512: bd4882a9c08e6a6eb14b7fb6366983db8581425b4949fea212785d34d8fad9e32fb81ca8c8cdbfb2c05ea394aaf5a746ba2cf16623795c7252c3bdb61d455f00

src/qt/bitcoin.h

@@ -68,7 +68,7 @@ public:
     /// Create options model
     void createOptionsModel(bool resetSettings);
     /// Initialize prune setting
-    void InitializePruneSetting(bool prune);
+    void InitPruneSetting(int64_t prune_MiB);
     /// Create main window
     void createWindow(const NetworkStyle *networkStyle);
     /// Create splash screen