CloneRepositoryDialog: design tweaks

[danirabbit]

Simplify things a bit:

• Set dialog modal. Happy to revert if it was intentional, but seems like we wouldn't want to spawn a bunch of the same dialog?
• Simplify placeholder text a big. example.com is reserved for examples
• Remove the folder image. This is a bit misleading as it looks like it's going to reflect the icon associated with the location but it always looks like the downloads folder
• Change the ellipsis icon to folder-open to better communicate what will happen when it's pressed
• Remove redundant headers
• Reduce margins
• Change from "URI" to "URL" since we specifically expect an http URL and not any resource location
• "Parent folder of clone" seems clunky. GNOME Builder just uses "Location" and that seems fine to me 🤷‍♀️