gtkplacessidebar: Fix commit f6870e5b79fd9ab

Commit f6870e5b79fd9ab introduced a some memory leaks and could be
improved in some areas.

Fix the memory leaks and apply the improvements.

https://bugzilla.gnome.org/show_bug.cgi?id=737983
Updates for 3.14.3
GtkBox: Ensure we only allow one center child

Glade was happily adding multiple center widgets. Oops.