Placement property not taken into account

The placement property is supposed to specify a hint as to where to place the driver.js popup relative to the target element (e.g. bottom left, top right, bottom center). It's only a hint, since driver.js will update the placement as the user scrolls the page, and will not use the placement hint if the popup would leave the screen.

The property can be set on the step XObject, but the property isn't passed to driver. (I forgot to implement it in the typescript code)

Alternatively, the property could be removed/hidden until it is implemented.