Improve kano-wifi password prompt and return to main menu

 * new class for a password prompt - each typed character is shown for .5 seconds then obscured by big dots
 * refactored code to unify password prompts (generic and hidden networks)
 * new password prompt is suggested if connection fails
 * fixed a bug in hidden networks, list of networks was not being displayed on return
Merge pull request #115 from KanoComputing/kanowifi-retry-password

Improve kano-wifi password prompt and return to main menu