[Folder View] improve label contrast against challenging backgrounds

Summary:
Currently the shadow behind Folder View items' labels is very subtle. This leads to poor contrast against wallpapers that are light-colored, visually busy, or both.

This patch attempts to improve the situation by tightening up the shadow so that it looks more like a subtle, tasteful outline.

BUG: 361228
FIXED-IN: 5.12.8

Test Plan:
Folder View items against various wallpapers:
{F6439459, size="full"}

{F6439458, size="full"}

{F6439463, size="full"}

{F6439457, size="full"}

{F6439456, size="full"}

{F6439455, size="full"}

{F6439454, size="full"}

{F6439453, size="full"}

{F6439452, size="full"}

{F6439451, size="full"}

Folder View items against the default wallpaper:
{F6439449, size="full"}

Reviewers: #plasma, hein, #vdg

Reviewed By: #plasma, hein

Subscribers: abetts, svenmauch, matheusm, filipf, emateli, mart, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16968