title: When user delete spreadsheet add-on, the document properties store added with the add-on continues to remain?
body: <p>I'm developing Google Sheets Add-on.</p>
  <p>I was just wandering that the document properties store is deleted when users uninstall the add-on.
  GAS don't have Trigger like 'onUninstalled'.
  If the properties continue to remain, I don't know where I write the program that delete them.</p>
  <p>Who knows my problem??</p>