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/VBWidgetType.h

@@ -0,0 +1,15 @@
+#pragma once
+
+enum class VBWidgetType {
+    None = 0,
+    GWidget,
+    GButton,
+    GLabel,
+    GSpinBox,
+    GTextEditor,
+    GProgressBar,
+    GCheckBox,
+    GScrollBar,
+    GGroupBox,
+    __Count
+};