Summary:
AbstractClient gains a new pure virtual killWindow method and this gets
implemented in ShellClient.

ShellClient performs the killing by sending a term signal to the
process. This can only work if the client connected through the socket
and didn't get a socketpair fd passed. In that case the pid is KWin's
and KWin doesn't want to terminate. Thus this is special handled to
destroy the connection instead.

In case terminating the process has no effect, the connection gets
destroyed after five seconds.

The KillWindow is adjusted to operate on AbstractClient instead of
Client.

This implements T4463.

Test Plan: Killed windows and auto test

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3370