Remove checks for NULL before g_free() and g_clear_object().
Merge check for NULL, freeing of pointer and its setting
to NULL by g_clear_pointer().
https://bugzilla.gnome.org/show_bug.cgi?id=733157
It seems that alternate implementations of GtkFileChooserWidget
never materialized. The split between GtkFileChooserWidget and
GtkFileChooserDefault is awkward. The immediate problem is that
it makes it difficult to document the keybinding signals. So it
makes sense to drop the abstraction and just have one thing.
https://bugzilla.gnome.org/show_bug.cgi?id=723157
