add PNG export

- [ ] Add export menu item, ⌘⇧E
- [ ] Save whole canvas. Default to transparent background.

Seems to be [very simple](http://stackoverflow.com/a/9545213/1460929):

```
NSBitmapImageRep* rep = [self bitmapImageRepForCachingDisplayInRect:self.bounds];
[self cacheDisplayInRect:self.bounds toBitmapImageRep:rep];
```

Start by locating the app's export menu and canvas rendering entry points; no file paths or tests are identified in the issue. Implement the PNG menu action with the ⌘⇧E shortcut, exporting the whole canvas with a transparent default background, then verify the resulting image includes the complete drawing.

Written by the indexing model from the issue text.