cinnamon-settings windows: Add minimum opacity setting for windows.

Depends on:
muffin           61096831db59763f6c121a3b86ccc2e8f8b0d29c (linuxmint/muffin#148)
cinnamon-desktop 8dacf5efad64038aefb2162e7bacbb1a080d7d7b (linuxmint/cinnamon-desktop#10)