Support user input in 2D content in 3D

Currently it is possible to embedded 2D Qt Quick Items into 3D scenes, but there is no way to forward input events (keyboard, mouse, touch) to those items.  There are some challenges related to this:

First the 2D items are rendered by Qt Quick, so we have limited information about the collision geometry (mesh) of these items.  That means that the items don't participate in the picking we currently have.  If they did those we would at least be able to tell where things were "touched".

Second the way input events work in Qt Quick, it assumes that they are going through QQuickWindow.  The 2D items that live in 3D though are managed by the 3D scene manager though instead, so they will need special translation as well as some kind of proxying mechanism.

Third, focus handling is going to be a challenge, just in general.