Implement show on screen edge for Wayland

Summary:
This change ports ScreenEdges to operate on AbstractClient instead of
Client. For this AbstractClient gained a new pure virtual method
showOnScreenEdge which is also implemented in ShellClient.

In ShellClient the functionality is bound for the case windows can
cover a panel. If triggered the panel gets raised again.

The auto hiding panel, though, is not yet implemented. For that the
protocol needs to be adjusted to give a hint to the compositor when to
hide and hint back to the panel when it was shown. This needs a change
in KWayland and thus is not 5.8 material.

Test Plan: See added test case

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2793