[PlasmaComponents Menu] Set WA_TranslucentBackground on the underlying QMenu
This ensures the menu can have nice round corners with Breeze and Oxygen. The same had already been done in ContainmentInterface's menu.
CHANGELOG: Fixed an issue where PlasmaComponents Menu would appear with broken corners
CCBUG: 381799
Differential Revision: https://phabricator.kde.org/D8965