Use the wpa_supplicant D-Bus interface when possible

[atheriel]

Following discussion in #293, I think we should look into @sebastianst's proposition to use the wpa_supplicant D-Bus interface. This would make the SSID code much less brittle, as well as moving us away from (expensive) polling. It would also fix issues like #247.

The drawback I can see is that this would tie us to wpa_supplicant. The proposed IWD block (see #257) might alleviate that worry, though.

[MaxVerevkin]

Is this still relevant?

moving us away from (expensive) polling

Netlink also provides a way to subscribe to network changes. This blog post can be used as an exaple: https://olegkutkov.me/2018/02/14/monitoring-linux-networking-state-using-netlink/