Prevent dragging QQC2 ComboBox contents outside menu

Summary: You should not be able to drag the items outside the menu, it looks weird. This fixes it.

Test Plan: Run the testComboBox.qml test and try dragging the items outside the menu. It should not be possible anymore.

Reviewers: Plasma, Frameworks, VDG, filipf, apol

Reviewed By: VDG, filipf, apol

Subscribers: VDG, Frameworks, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23285