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.cpp

@@ -0,0 +1,11 @@
+#include "VBProperty.h"
+
+VBProperty::VBProperty(const String& name, const GVariant& value)
+    : m_name(name)
+    , m_value(value)
+{
+}
+
+VBProperty::~VBProperty()
+{
+}