The alternative solution would be to call:
> image_new_dialog_set (dialog, NULL, NULL);
This would have reset to default template. But it's still not exactly like the
"new image" action which defaults to the active image's size if there is an
opened image.
In order to avoid complicating the code further, as well as code duplication,
hence code divergence, let's call the "image-new" action, making sure that this
button will always behave exactly like the "File > New" menu item.
Then since we need to process the return value of this dialog (either re-showing
the welcome dialog in case of new image creation cancelation, or destroying the
hidden welcome dialog otherwise), I check for the singleton pointer and connect
the handlers to it.
Moreover I made the "response" signal of ImageNewDialog be handled as
G_CONNECT_AFTER otherwise we nearly never had the possibility to handle its
responses properly (because the base handler was doing it first, then often
destroying the dialog).