Dynamically resizing windows requires private interface, please make public

In scopatz/xonsh#592, we want the amount of reserved space at the bottom of the prompt to be zero unless a tab event generates a menu of completion options. This can be done by swapping out the _height attribute of a Window instance. _height is a function that returns a LayoutDimension. This works great. The only thing that has me worried is that this is private interface. We do this swapping in the Completer as appropriate.

If there is a public interface, please let me know and I'll switch to that. If not please make this public to enable this much needed behavior. Note that we don't need / want the reserved space to drop back down after the completion. This is sufficient.

Start by reading the Window._height interface, LayoutDimension, and the Completer integration described in the issue; no file or test is named. Define and verify a public way to control reserved prompt space so it is zero by default, expands for completion menus, and does not shrink afterward.