Inserts this into the child widget list of parent.
It will be placed before next_sibling, or at the end if next_sibling is null.
After calling this function, `gtk_widget_get_next_sibling(widget)` will return next_sibling.
If parent is already set as the parent widget of this, this function can also be used to
reorder this in the child widget list of parent.
This API is primarily meant for widget implementations; if you are just using a widget, you *must* use its own API for adding children.
| this |
a `GtkWidget` |
| parent |
the parent `GtkWidget` to insert this into |
| next_sibling |
the new next sibling of this |