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

Depends on:
muffin           61096831db59763f6c121a3b86ccc2e8f8b0d29c
cinnamon-desktop 8dacf5efad64038aefb2162e7bacbb1a080d7d7b
Use % opacity for adjustment instead of 0-255
Merge branch 'min-window-opacity' of https://github.com/mtwebster/Cinnamon into mtwebster-min-window-opacity
Merge branch 'mtwebster-min-window-opacity'