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
logLevel = context
				.getInitParameter( ParameterAccessor.INIT_PARAM_LOG_LEVEL );
		
		scriptLibDir = processRealPath(
				context,
				context
						.getInitParameter( ParameterAccessor.INIT_PARAM_SCRIPTLIB_DIR ),
				IBirtConstants.DEFAULT_SCRIPTLIB_FOLDER, false );
		
		webAppLocale = getLocaleFromString( context
				.getInitParameter( INIT_PARAM_LOCALE ) );
		if ( webAppLocale == null )
			webAppLocale = Locale.getDefault( );
		isWorkingFolderAccessOnly = Boolean
				.valueOf(
						context
								.getInitParameter( INIT_PARAM_WORKING_FOLDER_ACCESS_ONLY ) )
				.booleanValue( );
		
		String s_maxRows = context.getInitParameter( INIT_PARAM_VIEWER_MAXROWS );
		try
		{
			maxRows = Integer.valueOf( s_maxRows ).intValue( );
		}
		catch ( NumberFormatException e )
		{
			maxRows = -1;
		}
		
		birtResourceFolder = processRealPath( context, context