[xwl] Generic X selections translation mechanism with Clipboard support

Summary:
In this patch an infrastructure is created to represent generic X selections
in a Wayland session and use them for data transfers between Xwayland windows
and Wayland native clients.

The central manager is the DataBridge class, in which Selection objects can be
created. This is hard-coded and such a Selection object persists until the end
of the session, so no arbitrary selections can be created on the fly. For now
the X Clipboard selection is supported, whose corresponding mechanism in the
Wayland protocol is just called Selection.

A Selection object listens for selection owner changes on the X side and for
similar events into the Wayland server interfaces. If a data provider is
available a selection source object is created by the Selection object. In case
data is requested on the other side, a data transfer is initialized by creating
a Transfer object. A Selection keeps track of all transfers and makes sure that
they are destroyed when they are finished or in case they idle because of
misbehaving clients.

The Clipboard class translates the X Clipboard via a proxy window. Selection
changes on the Wayland side are listened to through a new signal on the active
KWayland seat interface.

The previously used X clipboard syncer helper is disabled. The clipboard sync
autotest is changed to the new mechanism.

BUG: 394765
BUG: 395313

Test Plan: Manually and clipboard sync autotest.

Reviewers: KWin

Subscribers: zzag, graesslin, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D15061