Rate limit how often the current line is updated in python shell

* If there's a very tight loop there could be a _lot_ of updates to the point
  where the UI takes longer to redraw than the script does to step a line.
  Updating at 100Hz is still plenty fast.

qrenderdoc/Windows/PythonShell.h

@@ -160,6 +160,10 @@ private:
 
   static const int CURRENT_MARKER = 0;
 
+  bool m_CurLineDirty = false;
+  int m_CurLine = 0;
+  QTimer *m_CurLineTimer = NULL;
+
   PythonContext *interactiveContext = NULL, *scriptContext = NULL, *completionContext = NULL;
 
   QList<QString> history;