It looks like this got dropped during the move from autotools and never
restored. I can see why, since making it work wasn't a hugely fun task!
Notes on some less then obvious details:
* PlacesSidebar is private now and didn't seem to be to be particularly
easy to adapt to, so this moves to checking for it by name, not TYPE.
I couldn't find a (fast) better way; if you know how, please clean up
* added 2 casts to avoid warnings from the new type-propagating ref()
* GdkClipboard and GdkContentProvider need some properties dodged
* GtkToolItemGroup is gone
* fixed indentation and used TypeName:property-name syntax in a print()
ca94ff10ce