Make GtkIconInfo a GObject

Make GtkIconInfo a GObject

This is necessary in order to have async operations on it.
All the old copy/free functions keeps working, and g_boxed_copy on a GObject
also works, so this should be mostly compatible, but techncally its a minor
ABI break since the GType changes fundamental type. Changes like this has
happened before though, like with GVariant becomming its own fundamental
type.

https://bugzilla.gnome.org/show_bug.cgi?id=693802