Report windowing system

Summary:
Sometimes applications or Plasma have different code paths for X11 or Wayland.
Drkonqi reporting that saves one "question -> wait for reply -> period until reply
is seen"-cycle.

Test Plan: Try it in test mode

Reviewers: sitter

Reviewed By: sitter

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24890