The Online Accounts panel supports two commands: 'add'
and 'show-account'. The 'add' command must receive an
additional parameter, the 'provider' name, and an
optional 'preseed' parameter.
In the past, Settings would just push all the parameters
to a GVariant and pass to the panels. Now, however, it
skips the first one, making the parameter parsing code
in Online Accounts panel wrong.
This patch fix that by refactoring add_account() code,
and fixing the GVariant parameter parsing.
a9bf8b5b04