This patch also changes a few things about how new images are created by
making use of enums in the newImage interface. I think I've also
addressed the background opacity in a way that makes more sense and also
allows for users to tweak background opacity settings after creating a
new image. Lastly, while using RASTER or FILL backgrounds, if you create
more than one layer the background layer will start locked. This helps
to prevent accidental painting on initial starting background layers.