Fix wifi after changes from !425

I missed bringing in 1c4f14a0e50efcda09b4fd3106222e04e8f5e7f6 but even after that the wifi icon would be displayed too often since it did not take device availability into account but only whether nm considers wifi to be enabled. Fix this by

• cp 1c4f14a0e50efcda09b4fd3106222e04e8f5e7f6
• moving all the icon setting for disabled vs offline and hence the enabled calc into a single function
• fixing self->have_wifi_dev for the case when there's no dev present (which was broken all along)

This is something we need to fix before the next release.