Adding an rotation example

[Alberto Fanjul Alonso <albfan gnome org>]

Hi, I'm collaborating in a mockup designer called Akira https://github.com/akiraux and we are using goocanvas to draw objects.

I found little to none documentation to operate on objects by mouse, so find interesting adding an example:

https://gitlab.gnome.org/GNOME/goocanvas/merge_requests/1

Any review or suggestion is truly appreciated.

Even more, want to express my willing to help in any ongoing develop that could stabilize or improve goocanvas, just let me know if there's any roadmap