Move platform backends to backends directory

This improves file organization in kwin by putting backends in a single directory.

It also makes easier to discover kwin's low level components for new contributors because the plugins directory may come as the last place to look for. When one hears "plugin", the first thing that comes to mind is regular plugins, not low level backends.

In the future, I think it will be great to explore the possibility of decoupling wayland protocol bits from backends for a more future proof design.