[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