Core: Change IEditor::document() to const

All the implementations are const.

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

src/plugins/bineditor/bineditorplugin.cpp

@@ -394,7 +394,7 @@ public:
         delete m_widget;
     }
 
-    IDocument *document() override { return m_file; }
+    IDocument *document() const override { return m_file; }
 
     QWidget *toolBar() override { return m_toolBar; }