Called by the framework when it needs to draw the outline side bar. 
OnDrawOutlineBar is called when it needs to draw the outline side bar from the left (only when m_nOutlineMarginWidth is greater then 0 and outline bar is enabled). The default implementation fills the rectLeft rectangle with default editor background color and loops over all visible lines calling OnDrawOutlineButton to draw collapsible nodes buttons. 
- Parameters
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    | pDC | A pointer to a device context | 
    | rectLeft | Specifies the bounding rectangle of the line numbers bar area.  |