save user selected account

Use the GSettings to save the selected account and restore it on
launch. If no valid account is selected, then select the first
enabled RING account, then the first enabled SIP account, if any.

Change-Id: I3da2fecab49ee4c3deae47fe3c4ca106530e989c
Reviewed-by: Nicolas Jäger <nicolas.jager@savoirfairelinux.com>