Move X11 movingClient handling into a dedicated X11EventFilter

Summary:
Splits out the X11 specific window movement handling so that it's not
used in the Wayland case at runtime. As a nice side effect it
un-spaghetties the X11 event handler.

Test Plan:
Run nested KWin on Xephyr and nested KWin/Wayland to verify
that move/resize of X11 windows is still working

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7374