Add While background Layer as option in New window

[grofaty]

Tested in 1.7+r1120-202007292146~ubuntu18.04:

This is feature request, but would save me a lot of time if implemented.

I work a lot of time without any layers and most beginner users do the same.

Sample
1. Use some tool like Paintbrush and draw something on canvas.
2. Rectangle Select make some selection on canvas.
3. From toolbar user Cut.
What happens: Deleted rectangle background gets transparent "colour".
What most of the users expect: Deleted rectangle background gets "white" colour.

There may be many way how to "fix" this problem, but what about this one (to not break Pinta layers functionality):
1. In New window e.g. bellow Background option add new section: "Default deleted background colour" (probably something shorter would be nicer) and have three options (just like Background):
White, Background Color (if secondary colour is different then White) and Transparent.

What would happen if "White" is selected (in the actions of current Pinta 1.7-development words):
1. New image.
2. Menu Layer | New Layer.
3. Newly added layer is already marked. Use Paint Bucket and fill it with White colour.
4. Click on "Background" layer in Layers window.
TEST: Rectangle Select and use Cut from toolbar or press <Delete> button. It works, background colour is white.

Maybe to make is more beautiful rename "Layer 2" to something nicer e.g. "White Background".

Note: Additional feature request, when New window is selected respect this solution. So no need to constantly change selection.