Create a dedicated X11Filter for Client sync events and move it to X11 platform

Summary:
On Wayland we have the sync disabled as it doesn't work properly. This
allows us to also move the sync event handling into the X11 standalone
platform.

The code is slightly refactored: instead of passing the event to each
Client, we search for the matching Client. For that the SyncAlaram struct
is added to public section of Client. The method to handle the sync
doesn't need the event any more and is moved from events.cpp to
client.cpp.

Test Plan:
Run Xephyr+kwin_x11, resized a window and verified through
gdb breakpoint that the sync still works

Reviewers: #kwin, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7942