Standardize control widget padding

Nobody has claimed this yet.

This is coming up as I look to implement baseline alignment for widgets; several widgets have internal padding that complicates getting them to line up as expected with other widgets. The `RadioGroup`, for instance, adds 5 units of padding around each item in the group; putting a `RadioGroup` in a column with another widget requires and getting it to look right involves going and reading the source to figure out the correct padding value.

I'd like to try and simplify this as much as possible; this will mean removing any magic internal padding values, and trying to rationalize the rest of the padding values we use into a few items in the `Env`. In particular I would like to have a single standard 'inter-widget-padding' value.

Start by locating the RadioGroup implementation and the Env definitions, then inspect how internal widget padding is currently assigned across the controls mentioned by the issue. The work is done when magic internal padding is removed or rationalized into shared Env values, including a single standard inter-widget-padding value, while preserving the intended baseline alignment.

Written by the indexing model from the issue text.