[Menu] Harmonize openRelative placement

Use the enums in a consistent way to place the menu around the parent item.

Test Plan:
Tested with plasma-pa and task manager context menu.

Reviewers: Plasma, hein

Reviewed By: Plasma, hein

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4921