enable panning around the canvas

I think this is rather simple.

- [ ] Holding the spacebar changes the tool from drawing to panning until you release the key.
- [ ] Decouple `CanvasView` coordinate system from a `Canvas` coordinate system:
- [ ] When you pan, you add the drag/mouse-move vector `(Δx, Δy)` to a `scrollOffset`. Maybe part of the `CanvasView` itself, because it's a presentation detail.
- [ ] A `Canvas` representation then takes care of holding on to `Stroke`s and fetching them for a given `NSRect` partition of itself.

That's similar to drawing and scrolling tile maps in 2D games. `Canvas`is the map, `CanvasView` is the viewport or camera that moves around and asks the map for everything that's inside its boundary.

- [ ] Similarly, new strokes have to be translated from the `CanvasView` coordinates to the `Canvas` coordinate system.

Essentially, you just add the `scrollOffset: (Δx, Δy)` to the view coordinates and that should do the trick.