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

Beginne mit dem remotelist-Popup und dem vorhandenen fetch-Popup und verfolge dann die derzeit über die App zugänglichen Abläufe zum Erstellen, Bearbeiten und Löschen von Remotes. Als abgeschlossen gilt, dass das Popup das ausgewählte Remote abrufen, außerhalb von gitui geänderte Remotes widerspiegeln, die Remote-Abläufe enthalten und eingegebene URLs validieren kann.

Vom Indexierungsmodell aus dem Issue-Text verfasst.