testsuite/gtk/defaultvalue: Actually build...& fix

testsuite/gtk/defaultvalue: Actually build...& fix

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()