[feature] allow viewport being constrained by `root` element

[stefanprobst]

when passing in a root element via params, windows are appended to that element (as opposed to document.body), but the viewport constraining window movement is still derived from document.documentElement (see here)

it would be cool to (optionally) allow constraining window movement to the boundaries of the passed in root element (which would also require setting position: absolute on .winbox instead of position: fixed).

thanks!