gtype: add type declaration macros for headers

Add G_DECLARE_DERIVABLE_TYPE() and G_DECLARE_FINAL_TYPE() to allow
skipping almost all of the typical GObject boilerplate code.

These macros make some assumptions about GObject best practice that mean
that they may not be usable with older classes that have to preserve
API/ABI compatibility with a time before these practices existed.

https://bugzilla.gnome.org/show_bug.cgi?id=389585

docs/reference/gobject/gobject-sections.txt

@@ -103,6 +103,8 @@ g_type_ensure
 g_type_get_type_registration_serial
 g_type_get_instance_count
 
+G_DECLARE_FINAL_TYPE
+G_DECLARE_DERIVABLE_TYPE
 G_DEFINE_TYPE
 G_DEFINE_TYPE_WITH_PRIVATE
 G_DEFINE_TYPE_WITH_CODE