network: Introduce a new, rewritten Wi-Fi section

network: Introduce a new, rewritten Wi-Fi section

Remove the Wi-Fi chooser from the menu and put it in a dialog instead.
This frees up the submenu to simply have three items: an rfkill toggle,
a button to show the dialog, and a button to show network settings.

Ideally, we'd autodetect the "needs network" case by user initiation
and automatically show the dialog if needed, but lower-level plumbing
is neccessary, so the menu item to show the dialog is an acceptable
compromise instead.

This is a part of the new system status design, see
https://wiki.gnome.org/GnomeShell/Design/Guidelines/SystemStatus/
for design details.

https://bugzilla.gnome.org/show_bug.cgi?id=704670