Application: fix an error in dbus_unregister

We get a path too, and we need to pass it down to parent().
This was previous masked by running in DBus (and losing all stderr)
Add a global helper to get the application

Replace the "Gio.Application.get_default()" dance with a simple
getApp()
world: maintain a loading boolean property

The property is true any time we're updating an info, and is
notified accordingly.
Mark the application busy when the model is loading

In theory, this should show a spinner next to the app menu while
loading. In practice, it doesn't quite work...
Application: delay showing the window until the model is fully loaded

This way we avoid showing refresh icons every startup.
Mark that we support startup notification

Obviously, we're a normal gtk app...