#18086 RichText editor event tear down fix

Event handles generated in the onLoad function of RichText were not being 'owned' properly. I updated the mapper to return a list of handles instead of undefineds and created a handle passed to own that would call remove on each of the event handles

Added test to show RichText is no longer throwing errors when being destroyed

https://bugs.dojotoolkit.org/ticket/18086