Universal selection outline

[brandonreid]

The selection outline is obviously vital for communicating what is selected, but causes size changes on elements when it's rendered and can cause layout/placement issues. Instead, create one selection outline that's capable of detecting a selection and setting itself to surround and track the selected element. This will result in less elements to render and avoid placement issues on selected elements.

If doing this, the flow level artifacts no longer need to accommodate varying heights when selected. Other elements may also be able to forego accommodations.