ringmainwindow: update account status on signal

+ Update account status indicators when receiving accountStatusChanged.
+ Remove unused old_view variable.
+ Remove useless on_show_add_account function (replaced by
enterAccountCreationWizard)
+ Fix status label context switching

Change-Id: Iceae3da15d787540331b068c2eae2eebe95c3c43
Gitlab: #922
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>