[Touchpad KCM] New KWin Wayland version

Summary:
In order to work under KWin Wayland the Touchpad KCM needed adjustments.
In theory this should have meant only the creation of an additional backend.
But since a libinput based touchpad configuration utility needs to be in many
ways different than the legacy X version the decision was made to redesign
the user interface aswell.

Features:
- Changes the configuration of each connected touchpad individually
- Saves changes per device (through the KWin libinput backend) in config file
- Quick switch between devices via drop-down list
- Loads and removes devices on the fly on hotplugging events
- Visual indication if a property is not supported by a device
- Tooltips give additional informations about available options

Reviewers: #plasma, #kwin, graesslin, sebas

Subscribers: plasma-devel

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3617