Multires Modifier: Add ability to "delete" lowest resolution, make next level the new default

In ZBrush, it's not uncommon to "remove" the lowest resolution mesh in a subdivision series in order to have a new redefined "base resolution". This can be useful for changing your targeted "lowest quality" level while not losing all of your highest-level detail.

In blender, it's not possible to do this. The best work around is to duplicate the object, apply the modifier in the next-lowest resolution for viewport, readd the multires modifier, subdivide to the target highest level again, and then try to transfer the details by topology using the data transfer modifier. If that doesn't work (which happens on occasion) due to the indexing being incorrect, you will instead have to add a displace modifier followed by a shrink-wrap modifier, moving only in the -Z direction.

Obviously, these are a lot of steps to work around this limitation. It would be nice if we could simply delete the lowest level of detail in the multires mesh and also simplify the process of undoing it if possible.