Fix 1677632: newly drawn lines are joined on to existing lines and ducks wherever...

Fix 1677632: newly drawn lines are joined on to existing lines and ducks wherever possible:

  * closing a bline by drawing a new line from one end of it to the other end extends and loops the bline, as you would expect

  * connecting the end of one line to a duck in the middle of another (or even in the middle of the line itself) extends the existing line, and links the new end of the line to the duck

  * drawing a new line which starts and ends on the same duck now creates a new looped bline which is linked to the duck

git-svn-id: http://svn.voria.com/code@301 1f10aa63-cdf2-0310-b900-c93c546f37ac