Make it possible to autodetect the tablet mode

Summary:
At the moment it's broken for many of the laptops I've seen, this opens the possibility to start toying with them. The right fix would be to get the kernel to emit the right signals but I'm afraid for some hardware it will be close to impossible.
This includes a kconfig variable variable that will detect it as follows: if there's a touchscreen, when the touchpad gets removed we are on tablet mode (i.e. the pointing will be done using the finger), when it's connected back tablet mode will be restored.

Reviewers: #kwin, #plasma, davidedmundson

Reviewed By: #kwin, #plasma, davidedmundson

Subscribers: mart, davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19604