adjust dists to move picts into the mr distributions

This move moves a library from the plt distribution into the
mr distribution, which is a direction we do not generally
like to go, but the pict library is more core than slideshow
and the dependency that triggered this change is that the
2htdp/image library now interoperates with picts (hopefully
2htdp/universe will also work with them at some point).

So this teases out the pict part of slideshow (mostly,
but not completely the texpict collection) and moves only
that part to the mr distribution.