add requestToggleKeepAbove/below

Summary: client requests to toggle those states, to be used by libtaskmanager

Test Plan: setting keep above from the taskbar works

Reviewers: #plasma, hein, graesslin, #plasma_on_wayland

Reviewed By: #plasma, hein

Subscribers: graesslin, hein, plasma-devel, #frameworks

Tags: #frameworks, #plasma

Differential Revision: https://phabricator.kde.org/D5757