[libinput] Properly handle threaded Device creation

Summary:
Device is created from Connection::processEvents which is run in the
main gui thread, while Connection itself is in a different thread. Thus
passing *this* as parent is wrong.

This change removes the parent, moves the created Device into the
Connection thread and also deletes properly by using deleteLater.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1746