Move the new "default_new_layer_mode" APIs to the image...

...in both the core and libgimp.

Images now know what the default mode for new layers is:

- NORMAL for empty images
- NORMAL for images with any non-legacy layer
- NORMAL_LEGAVY for images with only legacy layers

This changes behavior when layers are created from the UI, but *also*
when created by plug-ins (yes there is a compat issue here):

- Most (all?) single-layer file importers now create NORMAL layers
- Screenshot, Webpage etc also create NORMAL layers

Scripts that create images from scratch (logos etc) should not be
affected because they usually have NORMAL_LEGACY hardcoded.

3rd party plug-ins and scripts will also behave old-style unless they
get ported to gimp_image_get_default_new_layer_mode().