Porting to GTK+ 3

[Murray Cumming <murrayc murrayc com>]

Shall we port to GTK+ 3, Damon? I could do that if it's wanted, or get
someone to do it.

This will be an ABI break, because GTK+ 3 breaks ABI for us. We probably
want to make goocanvas parallel-installable too.

As we are forced to do an ABI break, we could also take this opportunity
to make other changes that you didn't want to make yet because of ABI
stability.

-- 
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