Property to control multiview rendering

The environment variable (QT_QUICK3D_XR_MULTIVIEW) that allows opting in works fine and is going to stay available. It is also convenient for the implementation since it is there (set or not) when the XrView is constructed, and we do not have to bother with changes to the setting later on.

However, there needs to be an API of some sort. A property on XrView, in the absence of other options for Qt Quick (3D) in general. The challenges with this are:

• the user-provided property value may not be set, having still the default value, when the XrView initializes in the constructor (which is pretty early; should consider deferring, which may be a can of worms) If the handling of dynamic changes is implemented (see below), then this is fine, although suboptimal if we always initialize e.g. with multiview off, just so we can tear down and recreate stuff with multiview on a moment later.

• The property value can change later on. Involves at minimum tearing down the XR swapchains. Which is neither fast, and also should consider how this looks in a headset in VR when a running scene suddenly disappears then reappears? (is it disorienting, annoying, etc. to the user's eyes) On top, there is no good reason (is there?) for dynamically toggling this, except for development and testing purposes, so that one can put a checkbox or menu item to toggle multiview rendering on/off from within a scene.

• no other options, Qt Quick does not generally have the equivalent of project-wide settings (populated with user-set values already when the scene is being instantiated), like in other engines/tools