Fixed bug with undo/redo of scaling using scale tool: when scaling
canvases with canvases with canvases (or more complex), undo didn't
apply to the second inner level and within.
The issue was found in the visitTree method in CanvasModel.CanvasPanel,
where visiting wasn't forwarded to each child of a canvas panel.

Signed-off-by: Jakob Brandsborg Ehmsen <jakobehmsen.github@gmail.com>