Summary:
Change so we track track and set a DataSource instead of a DataDevice

This means we have to reverse a connection:
- we need to update Seat with our selection only when our selection is
received by the DataDeviceInterface

- we no longer need to track and watch a dataDevice for changes after
the seat emits selectionChange

Change so that we handle an AbstractDataSource. Meaning we can paste
from clipboard managers.

Testing done:
There is an existing xwayland-selections_test
This still passes.

Copied from:
wl-copy(wlr) to firefox (x)
firefox to wl-paste
firefox to kate (wayland)
kate to firefox

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: cblack, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29332