Port KCM menu away from PlasmaComponents

Summary:
Plasma's menu can't handle being in a nested window and opens in the
wrong place.

Porting to QQC resolves this, and we ideally shouldn't be using this
import within a KCM.

BUG: 414762

Test Plan:
Opened menu
It was in the right place
Clicking on delete still worked

Reviewers: #plasma, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26382