PixelPaint: Allow opening of files dropped onto application :^)

We can now drag-and-drop files onto PixelPaint to be able to open
them. Each dropped file opens in a separate editor (which is the
default behavior of Photoshop).

Userland/Applications/PixelPaint/MainWidget.h

@@ -43,6 +43,8 @@ private:
     ImageEditor* current_image_editor();
     ImageEditor& create_new_editor(NonnullRefPtr<Image>);
 
+    virtual void drop_event(GUI::DropEvent&) override;
+
     ProjectLoader m_loader;
 
     RefPtr<ToolboxWidget> m_toolbox;