Spin off scala.xml to library-xml.

Update build, partest accordingly.

The last vestige of xml remains in a comment in Predef and Context,
which may be useful for someone compiling quick with a compiler
that predates this commit and its parent.
Eclipse project for scala-library-xml.

Also added dependencies on scala-library-xml to other projects.
Unless you're using a version of Eclipse that includes the
scalac that does new-style xml (introduced a couple of commits ago),
you'll need to uncomment the line `val $scope = scala.xml.TopScope` in Predef.

Once 2.11.0-M4 is out, we can remove the dependencies on scala-library-xml,
except for the projects that really need it (partest and scaladoc).