[ResultDelegate] Fix bad text legiblity with certain 3rd party themes

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):

Arc Color (after):

McMojave (before):

McMojave (after):

Breeze (before):

Breeze (after):

Breeze Dark (before):

Breeze Dark (after):

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