Convert "background" to a drawable, not a style

[noahgibbs]

This would be a pretty big first issue -- but also a fun and useful one. There are a lot of steps here, but they're not hard steps. Feel free to ask for help if you take this on!

In Shoes3, "background" isn't a style on Stack or Flow. It's an object of its own. If you add it after text, it will cover it:

Shoes.app(width: 600, height: 50) do
  para "This is hidden under the background"
  background yellow
  para "This shows through on top"
end

So when converted to HTML, the background is just a colored div. Note that it's possible to have a gradient as the background. From example examples/legacy/simple/simple-calc:

background "#EEC".."#996", :curve => 5, :margin => 2

For this bug, we will need a "background" Drawable, both in lacci/lib/shoes/drawables and in lib/scarpe/wv. We should get rid of the "background" style on slot objects (like stacks and flows). You can find code for rendering a gradient to HTML in method background_style in scarpe-components/lib/scarpe/components/calzini/slots.rb -- Calzini is also where code to render the Background drawable to HTML should live. The rendering code should go away in calzini/slots.rb and instead be in whichever Calzini file you add background_element to.

The background should mostly be a div which defaults to height: 100% and width: 100%, though you can also explicitly set height and width on a background.

Right now it looks like image backgrounds aren't working properly. You don't need to fix them for this -- just keep the existing functionality, and move it into a drawable.