This fixes a potential leak of a PangoAttrList that is set when chaining
up to the parent get_preedit_string(). We check to see if the attr list
was created and reuse it instead of leaking the previous value.
If the parent get_preedit_string implementation returns a nonnull
zero-length string, then we ignore it, which is almost fine. We have to
free it, though.
Fixes#1174
When generating introspection data, we instantiate types without
calling gtk_init, so make sure that extension points are registered
before the type is trying to implement them.
