gtk/gtktoolbutton.c: Document why we need to still use G_TYPE_INSTANCE_GET_PRIVATE

This is the excellent explanation from Emmanuele at
https://gitlab.gnome.org/GNOME/gtk/merge_requests/402#note_361210:
"
Every time you instantiate a type, the instance_init() function is called for each
parent type T_p of your type T; to preserve invariants, the class pointer inside
the instance data is set to the parent type before each invocation, until you hit
your type T. This means that calling GET_CLASS() inside an instance_init() function
will give you a pointer to the class vtable for the parent type T_p while you're
iterating over parent types. What if you want to access the actual class vtable of
the type T, though? Well, you can because the actual signature of instance_init() is:

  void (* GInstanceInitFunc) (GTypeInstance *instance, gpointer g_class);

i.e. all instance_init() functions get passed the instance they are initialising
and the class vtable of the real type you're instantiating.

This is how GtkToolButton works: it "peeks ahead" at instance initialisation time,
to use the button_type class field of the actual type you're instantiating,
and calls g_object_new() with it to store the resulting object in its own private
data structure.

This whole contrived mechanism is needed to allow out-of-tree tool buttons to just
set the button type on their class init, and have their parent class create the
button they want, instead of asking all tool buttons to do this themselves and have
a virtual function called get_button() for GtkToolButton to call whenever it needs
to operate on the button instance.

Now we're coming to a close: we cannot use the G_DEFINE_TYPE macro because the
instance_init() function it creates internally will not pass the class pointer
to your custom instance_init(). Since we cannot use G_DEFINE_TYPE, we also cannot use
G_ADD_PRIVATE either.

This is the reason why, when I ported GTK 3 to the new private instance data structure
macros, I left GtkToolButton alone. I should have left a comment there, because @matthiasc
tried doing that as well, and then had to revert it in commit 1c4a7bd5. So: my bad,
sorry about that.

If we want to drop the G_TYPE_INSTANCE_GET_PRIVATE and the g_type_class_add_private() calls,
we cannot use G_DEFINE_TYPE, but what we can do is unrolling what the macros do themselves:

- add a global GtkToolButton_private_offset variable
- add a static inline gtk_tool_button_get_instance_private() that does return
(G_STRUCT_MEMBER_P (self, GtkToolButton_private_offset));
- call g_type_add_instance_private (g_define_type_id, sizeof (GtkToolButtonPrivate)) inside
gtk_tool_button_get_type() and store the result in GtkToolButton_private_offset
- replace g_type_class_add_private() inside gtk_tool_button_class_init() with
g_type_class_adjust_private_offset (klass, &GtkToolButton_private_offset)
"

gtk/gtktoolbutton.c

@@ -279,6 +279,10 @@ gtk_tool_button_init (GtkToolButton      *button,
 {
   GtkToolItem *toolitem = GTK_TOOL_ITEM (button);
 
+  /* We still need to use G_TYPE_INSTANCE_GET_PRIVATE() because GtkToolButton
+     need to access the class pointer inside instance_init
+     See a detailed explanation of this at
+     https://gitlab.gnome.org/GNOME/gtk/merge_requests/402#note_361210 */
   button->priv = G_TYPE_INSTANCE_GET_PRIVATE (button,
                                               GTK_TYPE_TOOL_BUTTON,
                                               GtkToolButtonPrivate);