[Folder View] Use an inactive highlight effect when something else has focus

Summary:
The theme SVG doesn't have a concept of "inactive selection highlight", so we need to
manufacture our own by reducing the opacity of the selection highlight.

Thanks to @broulik for suggesting the necessary code changes in the bug report.

FEATURE: 375501
FIXED-IN: 5.18.0

Test Plan: {F7743147}

Reviewers: #plasma, #vdg, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25160