image: Use FileList in ContentProvider

Makes dragging an image out of Loupe work in most cases.
There seems to be an issue with dragging from flatpak to host.
Removing the png from the provider seems to avoid gdk crashes
but not 100% sure that was the cause.

src/widgets/image_view.rs

@@ -126,7 +126,7 @@ mod imp {
                         gesture.set_state(gtk::EventSequenceState::Denied);
                         None
                     } else {
-                        obj.current_page().map(|p| p.content_provider())
+                        obj.current_page().and_then(|p| p.content_provider())
                     }
                 }),
             );