The UI definitions of the Wi-Fi devices currently contain many
widgets in the stack, such as the tower icon, the enable/disable
switch and the status.
In the new Wi-Fi panel, all those widgets will clutter the
interface and break the entire UI.
Fix that by splitting those widgets in two different containers:
1. The header_box container, with the menu button and the
enable/disable switch.
2. The center_box widget, with the title and status labels,
which will be consumed by the Wi-Fi panel to be the center
widget of the headerbar.
This commit also introduces two getters that expose those two
containers. With that, another load of code could be simplified.
https://bugzilla.gnome.org/show_bug.cgi?id=784818
