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/LabelWidget.inl

@@ -7,14 +7,6 @@
 
 namespace Nz
 {
-	inline void LabelWidget::UpdateText(const AbstractTextDrawer& drawer, float scale)
-	{
-		m_textSprite->Update(drawer, scale);
-
-		Nz::Vector2f size = Nz::Vector2f(m_textSprite->GetAABB().GetLengths());
-		SetMinimumSize(size);
-		SetPreferredSize(size);
-	}
 }
 
 #include <Nazara/Widgets/DebugOff.hpp>