Remotelist popup improvements

[robin-thoene]

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

[robin-thoene]

I would start working on this