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.
Currently, the entire screen will be repainted when using the qpainter
render backend. With this change, kwin will repaint only the dirty parts
on the screen.
