Replace mousedown overlay with pointer capture

[disconcision]

Simplifies our DOM structure and should actually make selection dragging work while your mouse pointer is outside the browser window, a current frustration.

https://developer.mozilla.org/en-US/docs/Web/API/Element/setPointerCapture

[cyrus-]

@dm0n3y maybe you can do this as part of the tylr selection work you're doing?