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/utility/ParameterAccessor.java
return columns;
	}
	/**
	 * Check whether enable svg support or not.
	 * 
	 * @param request
	 *            http request
	 * @return whether or not render content toolbar
	 */
	public static boolean getSVGFlag( HttpServletRequest request )
	{
		boolean svg = false;
		if ( "true".equalsIgnoreCase( getParameter( request, PARAM_SVG ) ) ) 
		{
			svg = true;
		}
		return svg;
	}
	/**
	 * Get web application locale.
	 * 
	 * @return report locale
	 */
	public static Locale getWebAppLocale( )
	{
		return webAppLocale;
	}
	/**
	 * This function is used to encode an ordinary string that may contain
	 * characters or more than one consecutive spaces for appropriate HTML