overview: Handle sessionMode.hasOverview changes

Currently we assume that either the initial sessionMode will have
the overview or none of the pushed modes - starting without the
overview and pushing a mode that adds it fails spectacularly.
However this is exactly what we are going to do when loading external
modes asynchronously - we'll initially use the default mode while
the modes are loading, and switch to the mode passed on the command
line when finished. So make sure that the overview UI gets initialized
properly in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=689304