Use G_TYPE_INSTANCE_GET_PRIVATE() instead
In the process, convert to using G_DEFINE_TYPE, too.
Fixes https://bugzilla.gnome.org/show_bug.cgi?id=609195
Signed-off-by: Javier Jardón <jjardon@gnome.org>
Either g_type_register_static_simple (used by G_DEFINE_TYPE_EXTENDED)
and G_IMPLEMENT_INTERFACE use automatic variables for GTypeInfo and
GInterfaceInfo structs, while tutorials and source code often use
static variables. This commit consistently adopts the former method.
https://bugzilla.gnome.org/show_bug.cgi?id=600158