The UI freezes for a moment when launching Settings -> Online Accounts
because g_application_register does slow I/O. After creating the
GApplication's GDBusActionGroup, it queries the list of remote actions
from the primary application instance over D-Bus to populate it.
Fortunately, it's not necessary to fill in the GDBusActionGroup for
activating a GAction. Therefore, this can be solved by directly using
a GDBusActionGroup instead of going via a launcher GApplication.

This does lose the platform data that the launcher GApplication would
have passed when invoking the remote GAction, but hopefully that
won't prove crucial.

The loss of error handling doesn't matter. g_application_register was
the only fallible step, and it could have only failed while querying
the list of actions from the primary, something that's not needed
anyway. If anything, it simplifies the code.

Fallout from 556a9f30