This implements the menu shadow changes proposed in D9549; now we use a combobox with five options: None, Small, Medium, Large, and Very Large.
Large is the new default value (64px window shadows, 16px menu/tooltip shadows), and implements what we recently changed the default to. Small is the old default value (16px window shadows, 12px menu/tooltip shadows) for people who preferred that.
I had a massive amount of help from @hpereiradacosta; in fact, probably 75% of these changes are his. It's a shame multiple authorship isn't possible. Hugo, feel free to commandeer the revision if you'd like the credit!