sizegroup: Construct the set of sizegroup peers as a hash table

sizegroup: Construct the set of sizegroup peers as a hash table

This way we don't need a marker on GtkWidgetParivate that needs to be
unset later, so we have all our data in the same place and can avoid
problems with reentrancy and shenanigans like that.

But the main reason I wrote that is cleaner code.