Polkit agent fixes + show dialog if admin user has empty password
Fix some bugs and usability issues in the polkit dialog:
- Make the "Authenticate" button insensitive by default until polkit asks for a password
- Make the "Authenticate" button insensitive if there's nothing in the password entry
- Also show the avatar for the root user (design input might be needed)
- Update the user name on user-changes
- Set key focus to the "Cancel" button if there's no password entry
- Fix a bug where the user avatar isn't visible on first run of the dialog
Including the changes from !30 (closed).
I know this is a bit of a "mixed-bag" MR that's not very beloved here, but it should still be reasonably quick to review and I figured splitting this up into smaller MRs would end us up with about 4 MRs dependent on each other.
The not completely sold on the last commit, but I can't think of a better way to prevent the visual annoyance.