RFC: Add some timespan-logging for potentially slow tasks

Add the startup duration of the shell to the startup-message, log a warning in case a component is slow to load (this one proved useful when debugging a slowly starting shell, it turned out initilizing a component triggered starting the gvfs systemd service), and finally log a warning in case an extension takes long to be initialized, enabled or disabled.

I'm not sure how much adding those metrics helps in practice (that's why this is marked as RFC), but at least the extension part is a good thing to have IMHO.