Core: Change IEditor::document() to const

All the implementations are const.

Change-Id: Ib9753fe764dd482d4f4392eec70878d42edc737a
Reviewed-by: hjk <hjk@qt.io>

src/plugins/texteditor/texteditor.cpp

@@ -7846,7 +7846,7 @@ void BaseTextEditor::addContext(Id id)
     m_context.add(id);
 }
 
-IDocument *BaseTextEditor::document()
+IDocument *BaseTextEditor::document() const
 {
     return textDocument();
 }