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
* Returns the dpi setting from http request
	 * 
	 * @param request
	 * @return
	 */
	public static Number getDpi( HttpServletRequest request )
	{
		String dpi = getParameter( request, PARAM_DPI );
		if ( dpi == null || dpi.trim( ).length( ) <= 0 )
			return null;
		return Integer.valueOf( dpi );
	}
	/**
	 * Returns the separator String
	 * 
	 * @param sep
	 * @return
	 */
	public static char getSep( HttpServletRequest request )
	{
		String sepKey = getParameter( request, PARAM_SEP );
		if ( sepKey == null )
			return DEFAULT_SEP;
		String key = "viewer.sep." + sepKey; 
		String sep = getInitProp( key );
		if ( sep.length( ) <= 0 )
			return DEFAULT_SEP;
		return sep.charAt( 0 );
	}
}