1) Selection masks now use KisProjectionLeaf hierarchy
   to embed themselves onto the top of the layers stack.
   That sounds a bit weird, but, yes, now the two
   hierarchies differ quite significantly.

2) KisImage::setOverlaySelectionMask() is used to mask one
   of the masks as an overlay.

3) KisNodeManager puts the mask into the overlay of the image
   as soon as the mask gets selected as a current layer.

4) KisProjectionLeaf now accesses the image using
   KisNodeGraphListener::graphOverlayNode(). That is not
   too nice, because it breaks the listener pattern, but
   that is the safest way to access the image from the
   internal nodes. We might want just rename this class :)