[server] Add support for setting the clipboard selection DataDeviceInterface manually

Summary:
So far SeatInterface handled automatically which DataDeviceInterface
holds the current clipboard selection. While this works fine and is
correct it doesn't support use cases like a clipboard manager where
the clipboard is hold by a different ClientConnection than the one
from the focused keyboard.

This change allows to manually set the selected DataDeviceInterface
to override the automatic selection, though the automatic selection
is still in place. Thus the next update of a selection will override
the manually set selection again.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D1972