_gtkTextChange copies the content (i.e. the internal gtkTextBuffer, so string, child widgets, tags, ...)
of a Text object in another Text object.
The two Text objects must be in the same subtype (easy or full).
Prototype : fun [ObjGtkText ObjGtkText] ObjGtkText
ObjGtkText : a destination ObjGtkText object.
ObjGtkText : a source ObjGtkText object.
Return ObjGtkText : the destination Text object
or nil if an error occurs.