Use cockpit.proxies and reduce the number of async methods

[imobachgs]

This PR refactors the network client to:

• use cockpit.proxies, making it easier to track the changes (using events on proxies) and to read the properties;
• reduce the number of async methods in the network client.

We can use the same approach for objects like /org/freedesktop/NetworkManager, but let's do it as a separate PR.

[coveralls]

Coverage increased (+0.4%) to 71.08% when pulling 6418032 on refactor/proxies into 70dbb4f on feature/wireless-support.

[coveralls]

Coverage increased (+0.4%) to 71.08% when pulling 54fd477 on refactor/proxies into 70dbb4f on feature/wireless-support.