Drop window style handling from ViCompletionController.
Stop setting the ViCompletionWindow as a floating panel.

This doesn't really seem to be working as expected anyway.
Make ViCompletionWindow a child window of its invoking text view's window.

We also adjust our positioning math to take into account this
new relationship, since the popup will now follow the window
when the window is moved.
When completion window is up, terminate completion when parent resigns main.
Add completion window as child after updating its bounds properly.

addChildWindow:ordered: seems to show the window briefly
even if it is hidden when the method is invoked. Thus, we
rewire things in the completion controller to allow us to set
up the bounds before adding the window as a child of the
invoker, so that we don't get a brief flash of the window in a
bad position.