improve visual feedback when dragging path curve

when changing a path curve directly by dragging it,
there was no real feedback for the user to see if
starting to drag would change the curve or doing a
rubberbanbd selection. now the cursor is being updated
to reflect if the mouse is onb a path segment. additionally
a status text is shown in the status bar.
i also refactored the corresponding code a little to reuse
the result of the path segment under the mouse cursor.