Remotelist popup improvements

# Summary

The PR #2350 closed the issue #2172 , but additional improvements that were delayed need to be made for the remote list functionality.

# Needed improvements

- an additional command is needed in the remotelist popup, to fetch the currently selected remote
- this should reuse the fetch popup
- the remotelist popup shall be able to display changes to the git remotes that had been done outside of the gitui tui app
- for example: the user added a remote using the standard cli -> the remote should appear in the remotelist popup of the gitui tui automatically
- the delete/create/edit functionality inside the `app` does not need to be globally available and therefore can be renamed to `start_remote_[OPERATION_NAME]_flow` and moved inside the `remotelist` popup
- the URL that is entered when adding/editing a remote should be validated

Empieza por el popup de remotelist y el popup de fetch existente, y luego sigue los flujos de creación, edición y eliminación de remotes que actualmente expone la aplicación. Se considera terminado cuando el popup pueda obtener el remote seleccionado, reflejar los remotes modificados fuera de gitui, abarcar los flujos de remotes y validar las URLs introducidas.

Escrito por el modelo de indexación a partir del texto del issue.