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/forms/intro.ui

@@ -7,7 +7,7 @@
     <x>0</x>
     <y>0</y>
     <width>674</width>
-    <height>415</height>
+    <height>447</height>
    </rect>
   </property>
   <property name="windowTitle">
@@ -210,16 +210,6 @@
      </property>
     </widget>
    </item>
-   <item>
-    <widget class="QCheckBox" name="prune">
-     <property name="toolTip">
-      <string>Reverting this setting requires re-downloading the entire blockchain. It is faster to download the full chain first and prune it later. Disables some advanced features.</string>
-     </property>
-     <property name="text">
-      <string></string>
-     </property>
-    </widget>
-   </item>
    <item>
     <widget class="QLabel" name="lblExplanation2">
      <property name="text">
@@ -240,6 +230,47 @@
      </property>
     </widget>
    </item>
+   <item>
+    <layout class="QHBoxLayout" name="pruneOptLayout">
+     <item>
+      <widget class="QCheckBox" name="prune">
+       <property name="text">
+        <string>Limit block chain storage to</string>
+       </property>
+       <property name="toolTip">
+        <string>Reverting this setting requires re-downloading the entire blockchain. It is faster to download the full chain first and prune it later. Disables some advanced features.</string>
+       </property>
+      </widget>
+     </item>
+     <item>
+      <widget class="QSpinBox" name="pruneGB">
+       <property name="suffix">
+        <string> GB</string>
+       </property>
+      </widget>
+     </item>
+     <item>
+      <widget class="QLabel" name="lblPruneSuffix">
+       <property name="buddy">
+        <cstring>pruneGB</cstring>
+       </property>
+      </widget>
+     </item>
+     <item>
+      <spacer name="horizontalSpacer_2">
+       <property name="orientation">
+        <enum>Qt::Horizontal</enum>
+       </property>
+       <property name="sizeHint" stdset="0">
+        <size>
+         <width>40</width>
+         <height>20</height>
+        </size>
+       </property>
+      </spacer>
+     </item>
+    </layout>
+   </item>
    <item>
     <spacer name="verticalSpacer">
      <property name="orientation">