Keep all touchpad QActions in the main thread

Summary:
Making QActions with the Connection as a parent is dangerous as it gets
moved to a new thread, moving all child objects to that thread.

KGlobalAccel and QAction aren't thread safe and we shouldn't be using it
in two threads. This was notably seen when runnng invokeShortcut over
DBus which then invokes it on the main thread. Something my laptop was
doing when I closed the lid.

This patch simply moves the code to the Input class, where we set up the
libinput connection.

Test Plan:
Closed lid, kwin_wayland was still there when I resumed
Set manual shortcut for toggling touchpad, that still worked

Reviewers: #plasma

Subscribers: plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D5733