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