wallpapers/image: lazy load image model and slide model

1. Make sure the image model and the slide model are only created
   when necessary, so the slide model will not consume resource when the
   image plugin is used, and vice versa.
2. When used in a config dialog, the model is created only after the
   component is completely created, which means all bindings are properly
   set. This avoids some unnecessary work.

CCBUG: 473798