Slow Welcome/Develop mode with more than a handful of sessions

When you have many sessions, performance of switching from another mode to welcome mode degrades (and probably also startup time).

I can reproduce a noticable delay with 15-20 sessions with 1-2 projects in them each.

Reason seems to be that the "projectPaths" are not fetched lazily only if the details of a session is opened, but it fetches them for all sessions when making the mode visible.