add basic support to show refactor markers

See BaseTextEditor::setRefactorMarkers(...). The markers support
a clicked signal via BaseTextEditor and tooltips. They feature
a cursor, but are always positioned at the end of the line (not block!).

As special gimmick they do extend the document width when they
are positioned outside the document area.

src/plugins/texteditor/basetextdocumentlayout.h

@@ -159,6 +159,11 @@ public:
     void emitDocumentSizeChanged() { emit documentSizeChanged(documentSize()); }
     int lastSaveRevision;
     bool hasMarks;
+
+    int m_requiredWidth;
+    void setRequiredWidth(int width);
+
+    QSizeF documentSize() const;
 };
 
 } // namespace TextEditor