PixelPaint: Keep a RefPtr to layer in LayerPropertiesWidget

Using a WeakPtr to keep a reference to the active layer caused it to
be destroyed when the last tab was closed, which made the
m_layer == layer check in set_layer() return early since it was
already null. Because of this the LayerPropertiesWidget was never
disabled.

Userland/Applications/PixelPaint/LayerPropertiesWidget.h

@@ -27,7 +27,7 @@ private:
     RefPtr<GUI::OpacitySlider> m_opacity_slider;
     RefPtr<GUI::TextBox> m_name_textbox;
 
-    WeakPtr<Layer> m_layer;
+    RefPtr<Layer> m_layer;
 };
 
 }