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