Bug#885781: petri-foo: Depends on libgnomecanvas

[trebmuh]

For information, there is someone working on porting petri-foo out of 
libgnomecanvas.
See :
- https://github.com/petri-foo/Petri-Foo/issues/37
- https://github.com/petri-foo/Petri-Foo/pull/40
Olivier