gui: Make Intro consistent with prune checkbox

When prune checkbox is toggled, the related text labels and the amount
of required space shown are updated (previously they were only updated
when the data directory was updated).

src/qt/intro.h

@@ -72,11 +72,12 @@ private:
     //! Total required space (in GB) depending on user choice (prune or not prune).
     int64_t m_required_space_gb{0};
     uint64_t m_bytes_available{0};
+    const int64_t m_prune_target_gb;
 
     void startThread();
     void checkPath(const QString &dataDir);
     QString getPathToCheck();
-    void UpdatePruneLabels(int64_t prune_target_gb);
+    void UpdatePruneLabels(bool prune_checked);
     void UpdateFreeSpaceLabel();
 
     friend class FreespaceChecker;