Create a dedicated X11EventFilter for the events used by RootInfo

Summary:
Splitting out the handling from events.cpp and moves it into a
dedicated class created together with RootInfo.

Test Plan:
Test case for NET window move which goes through this
code path still passes.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7808