[gui] add explicit prune setter

This makes it possible to enable pruning after the OptionsModel has been initialized and reset.

src/qt/bitcoin.h

@@ -67,6 +67,8 @@ public:
     void parameterSetup();
     /// Create options model
     void createOptionsModel(bool resetSettings);
+    /// Update prune value
+    void SetPrune(bool prune, bool force = false);
     /// Create main window
     void createWindow(const NetworkStyle *networkStyle);
     /// Create splash screen