Unfortunately there is no multi-select property panel control available among the native property components that Qlik has documented.
I have seen some extensions (ex vizlib) utilize custom property panel components but I don't think Qlik has ever officially supported that or released documentation on how to do that. I suspect the people that made the custom property panel components either reverse engineered the native ones or they have a contact at Qlik that gives them documentation that isn't available publicly to the rest of us.
