[wayland] Get rid of some duplicated code

Summary:
We duplicate ping code in a few places. This change introduces a dedicated
method for sending ping events to an xdg-toplevel. In long term, it can be
used for sending "test" ping messages at random interval.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26974