Support for syncing the clipboard from X11 to Wayland and vice versa

Summary:
The clipboard sync is done by a dedicated helper binary launched by
KWin. This helper binary is forced to xcb platform to piggy-back on
Qt's implementation of the X11 clipboard. In addition it implements
the Wayland clipboard - which is much simpler. Reading the Wayland
clipboard is based on the implementation in QtWayland.

KWin internally knows the DataDeviceInterface belonging to the helper
application. Whenever an xwayland client is focussed, this DataDevice
is allowed to set the selection and KWin manually updates the current
selection in the SeatInterface. By that the sync from X11 to Wayland
is implemented. When afterwards a Wayland client is selected, it's sent
the current selection which references the X clipboard and a data
transfer can be initiated in the normal Wayland way.

For the other direction KWin sends the current selection to the helper's
DataDevice whenever an xwayland window is focused. The helper application
reads the Wayland clipboard and sets it on the X11 clipboard. Thus the
Wayland clipboard is synced to X11.

The approach used here will also be useful for implementing a clipboard
manager (aka klipper).

Currently the implementation is not yet fully completed. We need to
make sure that the helper application gets restarted in case of a crash.

Test Plan: See added test case

Reviewers: Plasma on Wayland, KWin

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D1973