Inserts this into the child widget list of
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
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
This API is primarily meant for widget implementations; if you are just using a widget, you *must* use its own API for adding children.
the parent `GtkWidget` to insert this into
the new next sibling of this