[libinput] Add support for switch devices and events

Summary:
This change adds support for the switch devices introduces with libinput
1.7 (lid) and 1.9 (tablet mode). So far it's not yet used internally in
KWin, but only exposed through the Device and Events.

As KWin currently only requires libinput 1.5 and we are rather late in
the release cycle the new functionality is ifdef'ed. The requirement
will be raised once master is 5.13. It is already available on
build.kde.org, but e.g. Neon only has 1.6.

The switch events are interesting as they report whether the lid is
closed (might be interesting for e.g. powerdevil) and whether a
convertible is in tablet mode (supported for e.g. Lenovo Yogas with
recent kernel). This can be used by KWin internally to enable/disable
the virtual keyboard. And can be exposed globally to switch to Plasma
Mobile shell in future.

Test Plan:
Only through test case as my Lenovo Yoga uses Neon which has a
too old libinput

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, ...