Support Wi-Fi devices appearing dynamically

At Endless, we've seen several devices where the Wi-Fi driver comes up only after gnome-initial-setup has started. As a result, gnome-initial-setup believed there is no wireless hardware, and skipped the network page.

This branch teaches the network page to monitor for new devices being connected (and becoming managed by NetworkManager) and to react accordingly. I do not have a device affected by the original issue, so I have tested this entirely by unloading and reloading the kernel module for my laptop's Wi-Fi adapter.

This branch implements most of what's discussed on #82 (closed), with the exception of handling devices being disconnected. This is not a regression: the page keeps its old behaviour of just showing a “refreshing networks” spinner forever if the Wi-Fi device is disconnected. The page can be skipped in this case. While it would be nice to handle this properly, I don't have the time to work out all the edge cases right now.

Closes #82 (closed)