GApplication: don't fail if can't get session bus

If we can't get on the session bus, just behave like a normal non-unique
application.

This turns out to be remarkably easy to implement and lets us avoid
adding a 'dummy' backend.

Add a test for this case as well.

Idea from Zachary Dovel.

https://bugzilla.gnome.org/show_bug.cgi?id=651997