Scale icons with window size.

## Prerequisites
- [X] I have searched open and closed issues for duplicates.

## Feature
As per [proposed design improvements for switchboard](https://github.com/elementary/switchboard/issues/136#issuecomment-574471255), icons should scale up when the window becomes larger than a threshold size.

We should talk about that threshold: at what x,y is it appropriate to scale up the icons--or is it only appropriate when the window is fully maximized?

My original idea was for the icons to scale smoothly with window size, but that may be difficult to implement. For now, the plan is to swap the current 32px icons with [48px icons](https://github.com/elementary/icons/issues/876) at the threshold window size (and of course, back again, should the window decrease in size).