title: Bind a Line position to the position of an element in a usercontrol
body: <p>I have a canvas which contains specific usercontrols.</p>
  
  <p>At some point, I want to add a line to the canvas (in code) where one of the points is bound to the position of an element in a usercontrol. The usercontrol is dynamic, so elements can change position.</p>
  
  <p>The usercontrol contains a layout with grids, stackpanels etc. that the element can be located in.</p>
  
  <p>For the usercontrol, I'd get the Canvas.Left and Canvas.Top property,
  but since I don't want to bind to the usercontrol but to an element it contains,
  how would I do that ?
  (In a way that if the usercontrol's layout changes, the new position of the element is taken into account)</p>