online-accounts: Don't segfault if get_all_providers_cb is called during init
Due to an API bug in GNOME Online Accounts, the asynchronous goa_provider_get_all method doesn't accept a GCancellable argument. This makes it difficult to cancel an ongoing call when the CcGoaPanel gets destroyed.
This was hacked around by taking a reference on the panel for the duration of the call. Instead of cancelling a pending call on destruction, it would keep the panel alive until the call was over.
The hack does not consider the case get_all_providers_cb is called while the panel is still being initialized. E.g. this happen when create the panel passing the paramenters "add " because the constuct properties are set just after cc_goa_panel_init returns and because goa_provider_add_account will result in a call to g_main_context_iterate (caused by gtk_dialog_run).
In order to work around this we can call goa_provider_get_all in _constructed() after all construct properties have been set.