Widgets: Improve theme

- move simple styles classes ton their own file
- add button hovered texture
- add button hovered/pressed texture
- add label style

include/Nazara/Widgets/SimpleWidgetStyles.inl

@@ -0,0 +1,13 @@
+// Copyright (C) 2021 Jérôme "Lynix" Leclercq (lynix680@gmail.com)
+// This file is part of the "Nazara Engine - Widgets module"
+// For conditions of distribution and use, see copyright notice in Config.hpp
+
+#include <Nazara/Widgets/SimpleWidgetStyles.hpp>
+#include <Nazara/Widgets/WidgetTheme.hpp>
+#include <Nazara/Widgets/Debug.hpp>
+
+namespace Nz
+{
+}
+
+#include <Nazara/Widgets/DebugOff.hpp>