[server] Allow multiple touch interfaces per client

Summary:
The touch related code in the seat interface class has been for no apparent
reason very different to pointer and keyboard code.

This patch makes touch related code more similar and by that allows a client
to receive touch events through multiple interfaces.

Test Plan: Manually and auto tests still pass.

Reviewers: #kwin, #frameworks, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D15443