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/context/ViewerAttributeBean.java
if ( ParameterAccessor.isGetImageOperator( request ) )
		{
			return;
		}
		this.category = "BIRT"; 
		this.masterPageContent = ParameterAccessor
				.isMasterPageContent( request );
		this.isDesigner = ParameterAccessor.isDesigner( request );
		this.bookmark = ParameterAccessor.getBookmark( request );
		this.reportPage = ParameterAccessor.getPage( request );
		this.reportPageRange = ParameterAccessor.getPageRange( request );
		this.action = ParameterAccessor.getAction( request );
		
		if ( IBirtConstants.SERVLET_PATH_FRAMESET.equalsIgnoreCase( request
				.getServletPath( ) )
				|| IBirtConstants.SERVLET_PATH_DOWNLOAD
						.equalsIgnoreCase( request.getServletPath( ) ) )
		{
			this.reportDocumentName = ParameterAccessor.getReportDocument(
					request, null, true );
		}
		else
		{
			this.reportDocumentName = ParameterAccessor.getReportDocument(
					request, null, false );
		}
		this.reportDesignName = ParameterAccessor.getReport( request, null );
		
		this.format = ParameterAccessor.getFormat( request );
		if ( IBirtConstants.ACTION_PRINT.equalsIgnoreCase( action ) )