Alignments for RenderGroups

I quickly hacked in alignment for the LinearGroup.
The default behavior is aligned in the middle, like before, and the API does not break.

We should discuss the wording here, b/c *middle*, *left* and *right* makes sense only if the layout is vertical.

The example needs a little polish:
- [x] Animation on alignment toggle (Key C)
- [x] Canvas restriction seems broken. I painted the groups bounding box in gray for comparison