Achieve WCAG AA text contrast for labels
Part of plasma-desktop#149
Right now the labels use a variety of calculations to determine their appearance, none of them ideal from the perspective of WCAG contrast.
Instead, let's use the simpler and more common approach of 0.75 opacity, which achieves the following contrast ratios:
- 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.
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