Progressive session loader

[Slion]

Consider improving our session loader for a smoother experience. Ideally user should be able to see the number of tabs go up while the session is loading. That would actually slow the loading a bit but still provide a smoother user experience. Also user should be able to get faster access to the current tab in the loading session. The tricky part will be to manage user triggered tab creation and closure during loading. I guess we should just deny them