LibGUI: Use the new GUI::Tray widget in GUI::FilePicker

This removes all the extra entries from the keyboard focus chain and
makes FilePicker actually pleasant to navigate with the keyboard.

Userland/Libraries/LibGUI/FilePicker.h

@@ -61,7 +61,7 @@ private:
 
     struct CommonLocationButton {
         String path;
-        Button& button;
+        size_t tray_item_index { 0 };
     };
 
     RefPtr<MultiView> m_view;