Canvas: Image/Window unifying and allow using canvas inside canvas.

Canvas: Image/Window unifying and allow using canvas inside canvas.

 - Refactor and unify common functionality of canvas::Window and
   canvas::Image
 - Make canvas::Image actually work
 - Allow using canvases inside canvas::Image
   * Use new canvas:// "protocol" to allow using canvases in
     place of images
 - Prepare for categorizing canvases:
   * Move canvases to /canvas/by-index
   * Later support linking to other nodes in subbranches of
     /canvas