Python Editor: Introduce simple folding

Mainly go by code indentation, ignoring empty lines.
Handle comment blocks at 0 as a special case.

Change-Id: Ibe5bef7286c640a2eea8b50140dae256b6635a56
Reviewed-by: hjk <hjk@qt.io>

src/plugins/pythoneditor/pythonhighlighter.h

@@ -41,6 +41,9 @@ private:
     void highlightBlock(const QString &text) override;
     int highlightLine(const QString &text, int initialState);
     void highlightImport(Internal::Scanner &scanner);
+
+    int m_lastIndent = 0;
+    bool withinLicenseHeader = false;
 };
 
 } // namespace Internal