Introduce AbstractDataSource round the DataSourceInterface

Summary:
Clipboard managers and middle click paste are new protocols.

We want to be able to copy from a clipboard manager to a regular
clipboard and vice versa without duplicating loads of code.

If we support kliper's "syncronise contents of the clipboard and
selection" inside the compositor that would become an unmanageable amount
of combinations.

It also potentially allows the idea of our XWayland bridge not being a
wayland client and simplifying that code.

Test Plan: Unit test passes

Reviewers: #kwin

Subscribers: zzag

Differential Revision: https://phabricator.kde.org/D29329