VisualBuilder: Add a widget registry and a property class.

I need somewhere to centralize the knowledge about the different widget
types available. And VBProperty represents a property key/value of arbitrary
type (it uses a GVariant for the value.)

Applications/VisualBuilder/VBProperty.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <AK/AKString.h>
+#include <LibGUI/GVariant.h>
+
+class VBProperty {
+public:
+    VBProperty(const String& name, const GVariant& value);
+    ~VBProperty();
+
+    String name() const { return m_name; }
+
+    bool is_readonly() const { return m_readonly; }
+    void set_readonly(bool b) { m_readonly = b; }
+
+private:
+    String m_name;
+    GVariant m_value;
+    bool m_readonly { false };
+};