using QPainterPath for curve fitting - if possible


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1846 29e17f8d-cf37-471e-a288-72cc03a76eda
bezier interpolation for closed polygons added


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1847 29e17f8d-cf37-471e-a288-72cc03a76eda