WindowOverlay: fix title sizing

After the first time the title was placed, we were setting its width,
thus forcing get_preferred_width() to return that as the minimum and
natural width.
To workaround that, explicitly reset the width to -1, -1, causing
StLabel->get_preferred_width() to be called, which would give us a meaningful
value for minimum and natural width.

https://bugzilla.gnome.org/show_bug.cgi?id=688234