Feature request: spacing and margins as 'root' theme elements

[teunbrand]

Similar to how theme(line) is the root element for all other line elements, we might make spacing and margin root elements that all other spacings/margins inherit from.

I think this has two benefits:

• It would then become easy to make spacings and margins wider or narrower on the fly, instead of having to use e.g. theme_gray(base_size) to simulaneously scale all margins and spacings. The base_size argument also scales other stuff, like text, that you may not be interested in scaling.
• Developers can declare specialty theme elements that scale along with the base theme in register_theme_elements(). The current solution for this problem is to inherit a spacing from an unrelated theme element and pray it doesn't interact inconveniently.