Achieve WCAG AA text contrast for de-emphasized labels
Part of plasma-desktop#149
Right now several labels use enabled: false to become visually
de-emphasized, but this results in poor text readability and a WCAG
contrast ratio of only 1.62:1.
Changing that to the more standard approach of opacity: 0.75 increases
the ratio to the following:
- 5.73:1 on the opaque gray window background color
- 5.44:1 on a translucent background in Plasma with the current default wallpaper (though this will vary a bit with different backgrounds)
Now they meet WCAG AA.
The difference is somewhat dramatic, so here's a before-and-after:
| Before | After |
|---|---|
 |
 |