Add option to always invert text selection colours

The recent change of blending the background/foreground colours for text
selection doesn't work everywhere, users with vision impairment or
colour-blindness would probably find it hard to distinguish text selection
colours; and the current code can't possibly cover all variations of colour
schemes out there. The same goes with certain displays and viewing angles,
some display types are notoriously awful when viewed at an angle.

BUG: 435309
FIXED-IN: 21.08