_gtkBoxAdd add a Widget (Containers or not) in a Box
object.
Prototype : fun [ObjGtkBox * I I I I] ObjGtkBox
ObjGtkBox : a Box object.
* : a Widget object to add.
I : expand parameter : if the child Widget should (1) - or not (0) -
receive extra space when the parent resizes.
I : fill parameter : set if the extra space given to the child
Widget should be allocated to the child (1) or used as padding (0).
I : padding parameter : in pixels, set the extra space to put between the child
Widgets and its neighbors.
I : if the child Widget is added to the beginning (1), i.e.
in the top if the Box is vertical, in the left if the Box is horizontal,
or to the end (0), i.e. in the bottom if the Box is vertical, in the right
if the Box is horizontal.
Return ObjGtkBox : the same Box or nil if an error occurs.