dialog.Base: centering logic

it is time to get rid of:

    &.neo-centered {
        left      : 50%;
        top       : 50%;
        transform : translate(-50%, -50%);
    }

by default, a position: fixed div should already be centered.

however, the logic of animateShow() needs to grab the size of the offscreen rendered dialog & document body, to animate it to a calculated centered position.