Expose if the tablet mode switch is available

Summary:
expose in the libinput wrapper a property that tells whether
a tablet mode switch input device is present on the machine,
expose it trough dbus

Test Plan:
still not complete, I need a way to either access the connection
from TabletModeManager or setting to TabletModeManager from input.cpp

Reviewers: #kwin, #plasma, graesslin

Reviewed By: #kwin, #plasma, graesslin

Subscribers: graesslin, ngraham, davidedmundson, plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D9944