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
ParameterAccessor.PARAM_ISNULL )
								&& paramValue != null )
						{
							String nullParamName = getParameterName(
									(String) paramValue, parameterList );
							if ( nullParamName != null )
								this.configMap.put( nullParamName, null );
							continue;
						}
						
						else if ( ( displayTextParam = ParameterAccessor
								.isDisplayText( paramName ) ) != null )
						{
							paramName = getParameterName( displayTextParam,
									parameterList );
							if ( paramName != null )
							{
								this.displayTexts.put( paramName, paramValue );
							}
							continue;
						}
						
						paramName = getParameterName( paramName, parameterList );
						ParameterDefinition parameter = BirtUtility
								.findParameterDefinition( parameterList,
										paramName );
						if ( paramValue != null && parameter != null )
						{
							
							String typeVarName = configVar.getName( )