LibDraw: Put all classes in the Gfx namespace

I started adding things to a Draw namespace, but it somehow felt really
wrong seeing Draw::Rect and Draw::Bitmap, etc. So instead, let's rename
the library to LibGfx. :^)

Libraries/LibGUI/GSlider.h

@@ -91,7 +91,7 @@ private:
     bool m_knob_hovered { false };
     bool m_dragging { false };
     int m_drag_origin_value { 0 };
-    Point m_drag_origin;
+    Gfx::Point m_drag_origin;
     KnobSizeMode m_knob_size_mode { KnobSizeMode::Fixed };
     Orientation m_orientation { Orientation::Horizontal };
 };