Update event emulation to handle offscreen children

Update event emulation to handle offscreen children

We use the offscreen signals for getting parent, picking
children at a point and mapping coordinates between windows
embedding offscreens and offscreens.

This means we have two hierarchies more or less, one visible to apps via
the standard APIs and for drawing where the offscreens are their own
separate toplevels, and another one for event handling where embedded
offscreens appear as if they were children of the embedding window.