Add translucent background attribute to desktop icon popup menu

Summary:
This fixes the issue when menu transparency is enabled in the Breeze widget settings the context menu of desktop icons have no transparency.
Multiple users (including me) have reported that D14174 did not solve the problem.

BUG: 395262

Test Plan:
Before:
{F6253280}

After:
{F6253279}

Reviewers: hein, davidedmundson, #plasma

Reviewed By: davidedmundson, #plasma

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15435