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;
		
		String tempFolder = documentFolder + File.separator + sessionId;
		File file = new File( tempFolder );
		deleteDir( file );
		
		tempFolder = imageFolder + File.separator + sessionId;
		file = new File( tempFolder );
		deleteDir( file );
	}
	/**
	 * Returns if fit to page
	 * 
	 * @param request
	 * @return
	 */
	public static boolean isFitToPage( HttpServletRequest request )
	{
		String fitToPage = getParameter( request, PARAM_FIT_TO_PAGE );
		if ( "true".equalsIgnoreCase( fitToPage ) ) 
			return true;
		return false;
	}
	/**
	 * Returns if pagebreak pagination only
	 * 
	 * @param request
	 * @return
	 */
	public static boolean isPagebreakOnly( HttpServletRequest request )