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.isSupportedPrintOnServer )
				this.format = IBirtConstants.POSTSCRIPT_RENDER_FORMAT;
			else
				this.action = null;
		}
		this.maxRows = ParameterAccessor.getMaxRows( request );
		
		BirtResources.setLocale( ParameterAccessor.getLocale( request ) );
		
		ReportEngineService.getInstance( ).setMaxRows( this.maxRows );
		
		this.requestType = request
				.getHeader( ParameterAccessor.HEADER_REQUEST_TYPE );
		
		processReport( request );
		
		this.reportTitle = ParameterAccessor.getTitle( request );
		
		this.isShowTitle = ParameterAccessor.isShowTitle( request );
		
		this.isShowToolbar = ParameterAccessor.isShowToolbar( request );
		
		this.isShowNavigationbar = ParameterAccessor
				.isShowNavigationbar( request );
		
		this.moduleOptions = BirtUtility.getModuleOptions( request );
		this.reportDesignHandle = getDesignHandle( request );
		if ( this.reportDesignHandle == null )