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
this.missingParameter = false;
		else
			this.missingParameter = BirtUtility.validateParameters(
					parameterDefList, this.parametersAsString );
		
		this.isShowParameterPage = checkShowParameterPage( request );
		
		this.defaultValues = getDefaultValues( this.reportDesignHandle,
				parameterDefList, request, options );
		
		this.displayTexts = BirtUtility.getDisplayTexts( this.displayTexts,
				request );
		
		this.locParams = BirtUtility.getLocParams( this.locParams, request );
		
		this.parameters = (HashMap) getParsedParameters(
				this.reportDesignHandle, parameterDefList, request, options );
		
		this.parametersAsString = getParsedParametersAsStringWithDefaultValue(
				this.parametersAsString, parameterDefList, request, options );
	}
	/**
	 * Check whether show parameter page or not
	 * 
	 * @param request
	 * @return
	 */
	private boolean checkShowParameterPage( HttpServletRequest request )
	{
		if ( !ParameterAccessor.HEADER_REQUEST_TYPE_SOAP