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because each item has a shell now. Keep the parameter in the virtual
functions though because that saves a lot of code in the subclasses.

and pass it to all constructors. The GimpDisplayShell is needed
because items are going to become more powerful soon.

The code is almost identical to the group stroking feature.

In order to avoid visually overlapping lines:

- GimpCanvasItem got a "suspend_stroking" API that allows to skip
  stroking and instead starts a new sub-path
- GimpCanvasGroup got a "group_stroking" API that sets suspend_stroking
  on all its children and does one stroke after calling all items'
  draw().

It can draw them all with one call to draw() and returns a union of
their extents via get_extents().