Add option to enable Light Tree only for final render

Add a little option next to the Light Tree checkbox in render properties, to enable Light Tree only when doing a final render (F12).

This is helpful because in some scenes, Light Tree makes the "updating lights" process take up to a minute (as opposed to half a second), making viewport rendered visualization almost infeasible. This option would save one step before rendering (checking whether to enable light tree or not). It would be set and forget, instead of having to switch it constantly between viewport previews and renders.