wire up requestToggleKeepAbove/below

Summary: cobbect requestToggleKeepAbove and below with libtaskmanager

Test Plan: setting keep above from taskbar works

Reviewers: Plasma, hein, graesslin, Plasma on Wayland

Reviewed By: Plasma, hein, graesslin

Subscribers: graesslin, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5758