Draft: layout: Add check about whether session should start on Overview

In situations that there are UI applications that are meant to be started together with the session, it is undesirable to boot into the overview.

In ideal situations, we would have session management infrastructure in place that would allow GNOME Shell to find out there's UI services to be brought up in the startup phase, and be able to decide window positions and workspaces beforehand.

But we don't have any of that. Add a sneaky file existence test so the behavior can be tweaked by savvy users and e.g. installers. The hope being that this is a temporary solution till we have the ideal framework in place.

Related: #3854