[GTK4] Stop using `gtk_widget_set_parent`?

[jonahgraham]

While working on #2819 I see that Button uses gtk_widget_set_parent:

eclipse.platform.swt/bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Button.java

Line 392 in a0d6ffc

GTK.gtk_widget_set_parent(boxHandle, handle);

However gtk_widget_set_parent is documented with this note:

This function is useful only when implementing subclasses of GtkWidget.

In gtk_button_set_child, which is the correct method for GtkButton to change parent/child relationship, it calls gtk_widget_set_parent here.

This means there is some stuff that gtk_button_set_child does that we are missing in SWT's implementation. However, I don't know if there are any user visible effects of using this method. I plan to return to this later, once #2819 is done.