_gtkWindowMovePredefined places a Window object
to a predefined position on the screen (if the Window Manager allows this).
This function should be put before a Show/ShowAll action.
Prototype : fun [ObjGtkWindow I] ObjGtkWindow
ObjGtkWindow : a Window object.
I : a predefined position. One only value :
WIN_POS_CENTER (on the screen)
WIN_POS_MOUSE (where the pointer is)
WIN_POS_CENTER_ALWAYS
WIN_POS_CENTER_ON_PARENT
another value is no effect
Return ObjGtkWindow : the same Window object
or nil if an error occurs.