_gtkLabelEllipsizemodeSet defines the mode used to ellipsize
(add an ellipsis: "...") to the text if there is not enough space to render
the entire string in a Label object.
Prototype : fun [ObjGtkLabel I] ObjGtkLabel
ObjGtkLabel : the Label object.
I : the ellipsize mode :
ELLIPSIZE_NONE : no ellipsization to do
ELLIPSIZE_START : omit characters at the start of the text
ELLIPSIZE_MIDDLE : omit characters in the middle of the text
ELLIPSIZE_END : omit characters at the end of the text
Return ObjGtkLabel : the same Label object
or nil if an error occurs.