Move decoration pixmap handling from Client to PaintRedirector

The only task of the PaintRedirector is to redirect the painting of the
window decorations into Pixmaps. So it should actually do this by also
handling the four pixmaps for the decoration. This simplifies the code
as all the logic concerning redirecting the painting is now grouped
together.

Furthermore the PaintRedirector is now a child of the decoration widget,
which means it gets automatically destroyed whenever the decoration is
destroyed - the Client does not have to care about it.

Also the PaintRedirector gets only created if the Compositor is active as
it is not needed in the non-compositing case.

REVIEW: 106620