Add minimum opacity setting for user-adjusted window opacity (mouse wheel and
keybindings)

Depends on cinnamon-desktop 8dacf5efad64038aefb2162e7bacbb1a080d7d7b
Read minimum window opacity as a 0-100 value, convert it to 0-255.
Merge branch 'min-window-opacity' of https://github.com/mtwebster/muffin into mtwebster-min-window-opacity

Conflicts:
	src/core/prefs.c
Merge branch 'mtwebster-min-window-opacity'