[Folder View] Add subtle shadows to icons

Summary:
Folder View items currently have shadows behind their labels, but not their icons.

This patch adds a very subtle shadow for the icons as well, which improves contrast
for some backgrounds and just looks classy IMO. :)

FEATURE: 411607
FIXED-IN: 5.17.0

Test Plan:
Before: {F7334103, size=full}
After: {F7334104, size=full}

Reviewers: hein, #plasma, #vdg, filipf, ndavis

Reviewed By: #vdg, filipf, ndavis

Subscribers: broulik, GB_2, cfeck, ndavis, filipf, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23803