cubic splines almost done


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1903 29e17f8d-cf37-471e-a288-72cc03a76eda
splineeditor adopted


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1904 29e17f8d-cf37-471e-a288-72cc03a76eda