Summary:
This is a fix for D25043.

Using `disabledTextColor` results in bad text legibility with Plasma themes that use filled-style highlight effects.

If colors are prefered over opacity as means of achieving text shading then we need to mix `disabledTextColor` with `textColor` on selection.

This creates an effect that is comparable to the `opacity: isCurrent? 0.8 : 0.6` code that is used throughout Plasma for subtitles.

Test Plan:
Only test with master.

Arc Color (before):
{F7770216}

Arc Color (after):
{F7770217}

McMojave (before):
{F7770218}

McMojave (after):
{F7770219}

Breeze (before):
{F7770215}

Breeze (after):
{F7770221}

Breeze Dark (before):
{F7770222}

Breeze Dark (after):
{F7770223}

Reviewers: #plasma, #vdg, ngraham, broulik

Reviewed By: #plasma, #vdg, ngraham, broulik

Subscribers: ngraham, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25374