Adds a new "Ui Asset Shelf" template to the script editor, demonstrating
how scripts can register own asset shelves. The asset shelf is designed
as a new standard UI element that add-ons, application templates and the
like can use, so this is useful. It's also useful as a quick way to test
and customize the asset shelf.
The script is only available in the UI if the "Asset Shelf" experimental
feature is enabled in the Preferences.
