Add background color for splash screen.

[marcoscaceres]

Conditional on: #387

If we remove image object's background_color, we need a way of coloring the backgrounds of splash screens.

[mounirlamouri]

I think I would prefer if the website could tell us the background colour of its content in the Manifest so we can construct a splashscreen that it smooth. The difference is mostly semantic though.

[marcoscaceres]

@mounirlamouri, you still need the background color for the splash screen image, no? Working assumption is that there will be a transparent png being painted on some given color.

[marcoscaceres]

This is handled by https://w3c.github.io/manifest/#background_color-member