NetworkManager VPN plugin auth support

This adds initial support for authenticating NM vpn connections

Also:

• split out the auth bits from wifimanager into a separate class
• remove unnecessary passing around of nm client and late initialization of nm-agent
• fix cancellation of nm-agent setup (should fix #673 (closed))
• make auth-prompt robust against reuse so overlapping requests don't cause trouble

The basic code was sitting around for some time already but I didn't want to send it out without the above cleaned up so we can add more prompts without much trouble. The VPN prompts are the most complex ones as far as I can tell. CI tests would be rather involved since we'd need #322 and a VPN plugin setup so I skipped those.

I'd welcome testing with other VPN plugins besides openvpn and iodine.

There's no VPN quick setting or indicator yet (#631 (closed)). Adding that is rather simple now though. For testing one can enable/disable VPN connections via g-c-c or nmcli.

Closes: #630 (closed)