todoist-preferences-panel: fix signal connect for add_button

Since, add_account_button_clicked only need the data,
that is GtdTodoistPreferencesPanel rather than instance
add_button, it should be g_signal_connect_swapped.
Currently with just g_signal_connect the callback
function will not work properly since data won't be
correctly passed.

https://bugzilla.gnome.org/show_bug.cgi?id=772278