goocanvas: replace sample scalability-demo by GI

I wrote notes in source code.

## startup time (COLS=15, ROWS=10)
### gi
Total items: 30150
Create Canvas Time Used: 33.81387643
Update Canvas Time Used: 0.274296273

### no-gi
Total items: 30150
Create Canvas Time Used: 3.80782131
Update Canvas Time Used: 0.267654617