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
{
		String pagebreakOnly = getParameter( request, PARAM_PAGEBREAK_ONLY );
		if ( "true".equalsIgnoreCase( pagebreakOnly ) ) 
			return true;
		return false;
	}
	/**
	 * Returns how to open attachment( inline or attachment )
	 * 
	 * @param request
	 * @return
	 */
	public static String getOpenType( HttpServletRequest request )
	{
		if ( "true".equalsIgnoreCase( getParameter( request, 
				PARAM_AS_ATTACHMENT ) ) )
			return IBirtConstants.OPEN_TYPE_ATTACHMENT;
		return IBirtConstants.OPEN_TYPE_INLINE;
	}
	/**
	 * Returns action name
	 * 
	 * @param request
	 * @return
	 */
	public static String getAction( HttpServletRequest request )
	{
		return getParameter( request, PARAM_ACTION );
	}
	/**