_gtkTextStringSet sets the entire text content to a Text object.
The old one, if any, is overwritten. In deep, other contents such as image widgets
can be added in a Text (Full) object.
Other functions allow to set this to some parts of a text.
Prototype : fun [ObjGtkText S] ObjGtkText
ObjGtkText : a Text object.
S : the new content.
Return ObjGtkText : the same object
or nil if an error occurs.
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