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