Split libinput and legacy Mouse and Touchpad KCMs

The mouse and touchpad KCMs currently support three different "platforms": KWin Wayland Libinput, X11 Libinput, X11 Synaptics/XLib.

Wayland Libinput and X11 Libinput share mostly the same UI, which is using QML. However the Synaptics UI is completely different and using Widgets.

Supporting all of that from the same KCMs makes them massively complex. They have internal backend systems and the KCM is a hybrid of QML and widgets, with all the challenges that brings.

This is not sustainable. We should split the KCMs into a Libinput variant and a legacy variant.

The challenge here is that the current thing handles switching between libinput and legacy at runtime. Would it be fine if it was a compile-time option?