VisualBuilder: Hook up everything needed for widget property editing.

It's now possible to edit widget properties inline in the properties window.
We're currently relying on the basic GVariant conversion functions to do
all the "parsing" but that's not gonna be good enough.

LibGUI/GSpinBox.h

@@ -15,6 +15,8 @@ public:
 
     int min() const { return m_min; }
     int max() const { return m_max; }
+    void set_min(int min) { set_range(min, max()); }
+    void set_max(int max) { set_range(min(), max); }
     void set_range(int min, int max);
 
     Function<void(int value)> on_change;