gtkbuilder: check for existing object before extending template

gtkbuilder: check for existing object before extending template

If gtk_builder_expose_object() is called twice with the same name, it will
result in a g_critical(). This improves that situation by checking for the
object before exposing additional times.

This turns out to be handy in situations where templates are expanded
multiple times, such as application-side implementations of UI merging.