Bug 973466 - [settings] refactor wifi panel with AMD pattern

This patch is still under WIP.
The root panel should be notified with the change of the wifi status instead of providing the status. We may have other panels displaying the status in the future, in that cases those panels will depend on the root panel instead of `WifiContext` and which makes less sense IMHO.

I noticed that the string was used in wifi panel, in that case, the panel should compose the string by itself.