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

This patch is still under WIP.
I added one more event called `wifiStatusTextChange` on WifiContext ! For any panel, if they just want to reflect latest status wifi, they don't need to do any trivial stuffs any more ! WifiContext would handle all of them internally (including l10n change) !

I think this new interface would make it more clear and easy to use, WDYT @crh0716  ?