decoder: Use GFile instead of PathBuf

This allows to show images from locations that don't
have a representation with path.

This also works around not being able to read files in trash:///

Closes #56

Part-of: <https://gitlab.gnome.org/Incubator/loupe/-/merge_requests/174>

src/application.rs

@@ -73,7 +73,7 @@ mod imp {
             let application = self.obj();
             for file in files {
                 let win = LpWindow::new(&*application);
-                win.set_image_from_file(&file);
+                win.set_image_from_file(file);
             }
         }
     }