network: Center "Wi-Fi hotspot" properties
network: Fix warning on startup with hotspot enabled

There's no back button anymore, so we shouldn't try to focus it.

Gtk-CRITICAL **: gtk_widget_grab_focus: assertion 'GTK_IS_WIDGET (widget)' failed
network: Add hotspot_switch struct member

So we can access it directly when needed.
network: Show an error when we fail to turn off hotspot

And reset the switch as well. I don't know in which circumstance it
could happen, but it's all we can do for now.

https://bugzilla.gnome.org/show_bug.cgi?id=723643