Bug 183919 [Enhancement] add a switch to determine whether to clean viewer image folder when session expires
in current viewer implementation, file in image folder is deleted when its corresponding session expires, could viewer add a switch to allow users to decide whether this deletion operation should be performed.
viewer/org.eclipse.birt.report.viewer/birt/WEB-INF/classes/org/eclipse/birt/report/context/ViewerAttributeBean.java
String curName = null;
				if ( parameter != null && parameter.getName( ) != null )
				{
					curName = parameter.getName( ) + "_" + parameter.getId( ); 
				}
				
				if ( curName != null
						&& curName.equalsIgnoreCase( configVarName ) )
				{
					paramName = parameter.getName( );
					break;
				}
			}
		}
		return paramName;
	}
	/**
	 * Returns the report design handle
	 * 
	 * @param request
	 * @throws Exception
	 * @return Report Design Handle
	 */
	protected IViewerReportDesignHandle getDesignHandle(
			HttpServletRequest request ) throws Exception
	{
		IViewerReportDesignHandle design = null;
		IReportRunnable reportRunnable = null;
		IReportDocument reportDocumentInstance = null;