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
.equalsIgnoreCase( this.requestType )
				&& !IBirtConstants.SERVLET_PATH_DOWNLOAD
						.equalsIgnoreCase( request.getServletPath( ) ) )
		{
			String showParameterPage = ParameterAccessor
					.getShowParameterPage( request );
			if ( "false".equalsIgnoreCase( showParameterPage ) ) 
			{
				return false;
			}
			if ( "true".equalsIgnoreCase( showParameterPage ) ) 
			{
				return true;
			}
		}
		return this.missingParameter;
	}
	/**
	 * parse paramenters from config file.
	 * 
	 * @param request
	 *            HttpServletRequest
	 * @param parameterList
	 *            Collection
	 * @return
	 */
	protected void parseConfigVars( HttpServletRequest request,
			Collection parameterList )
	{
		this.configMap = new HashMap( );