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 <provider>" 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.

GNOME/gnome-control-center#401
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