kcms/rules: add warning for low opacity

Once upon a time, in a moment of acute stupidity, I accidentally created a window rule making all windows completely transparent. I meant to go for 0% transparency when really the slider said 0% opacity. And once you've made the very window to change this setting invisible, the only way to fix that mistake is to log into a TTY or different DE and find the config file to edit manually. Since I've since seen several KRedditors fall into the same trap:

Show a warning when active or inactive opacity is set to <= 15% < 25%.