Improve StatusLight class and Pane linking

 - Link lines now have differently styled end nodes
 - The appearannce of the end node on a link line now communicates whether
   a link is possible with the hovered pane
 - Previewing the link using by hovering over the link icon now shows
   links recursively. The line pointing to the parentmost pane has a
   larger end node to indicate its status.
 - Link lines and link display lines are now held in a container exclusive
   to them, managed by an Editor instance
 - Add Editor.getPaneAtCoordinate() to allow the fetching of panes using
   screen coordinates
 - Panes no longer request links by firing custom events, instead
   interacting with each other directly. If the link needs to be made
   using a wrapper element or screen position, the Editor API is used.