Add a preferences toggle for forcing material preview and load UI off by default.

In the "Blender 4.0 is HUGE" livestream Pablo mentioned a setting similar to "Load UI" but for material preview. I have a slightly different suggestion. We could use a setting in preferences that toggles each of these settings on or off. One for UI and one for forcing material preview off by default.

I bring this up because I've wasted a ton of time waiting for shaders to load for no reason, or to reload a blend file, then load the one I want again with "Load UI" disabled. For Load UI, by having it on by default it's pretty much guaranteed if you don't often open someone else's blend file that you will completely forget to do it when you actually need it and the same thing goes for material preview.