Move XRandR event filter into XRandRScreens

Summary:
The code in events.cpp was problematic as it was called in a Wayland
session. So KWin changed outputs, this gets mirrored to XWayland and
then KWin reacted on the XRandR event and might have even changed the
refresh rate due to that - bad idea.

This change moves the code into the already existing X11EventFilter for
XRandR events in XRandRScreens.

Test Plan: Run kwin_x11 in gdb on Xephyr, breakpoint in new code and triggered XRandR event

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7654