[FTU] Bug 811149 - Better feedback for connecting to WiFi

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Show message when no networks
Updated sprites for different wifi states
Refactor wifi UI outside general UI
Icon animation while connecting to wifi
Message overlay while scanning networks
Join button disabled when password too short
Merge pull request #8084 from fcampo/wifiFeedback

[FTU] Bug 811149 - Better feedback for connecting to WiFi (r=fbsc,igonzaleznicolas,l10n)