ENH: Change 'AddMarkup' to also return the markupIndex on success.

This commit updates the function 'AddMarkup' to behave similarly
to 'AddMarkupWithNPoints' function.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23671 3bd1e089-480b-0410-8dfb-8563597acbee