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
* true to render the reportlet.
	 * 
	 * @param request
	 * @return true for render the reprtlet based on the instance id.
	 */
	public static boolean isIidReportlet( HttpServletRequest request )
	{
		String instanceId = getParameter( request, PARAM_INSTANCEID );
		return instanceId != null && instanceId.length( ) > 0;
	}
	/**
	 * Check whether the viewer allows master page content or not.
	 * 
	 * @param request
	 * @return
	 */
	public static boolean isMasterPageContent( HttpServletRequest request )
	{
		boolean isMasterPageContent = true;
		if ( "false".equalsIgnoreCase( getParameter( request, PARAM_MASTERPAGE ) ) ) 
		{
			isMasterPageContent = false;
		}
		return isMasterPageContent;
	}
	/**
	 * Check whether report design will overwrite report doc or not.
	 * 
	 * @param request
	 * @return