Add option to display Item Actions in dropdown menu

[Flo0807]

If you have a certain number of item action in your resource, you might want to list them in a dropdown so that the table looks clean.

There should be an option that can be set individually on each item action to display it in a dropdown. If at least one action wants to be displayed in a dropdown, a dropdown is automatically added to each table row (if the row displays the action).

We need to discuss whether we intend to make the dropdown (e.g., the icon of it) customizable and if the option is only for table rows. Users might want to have a dropdown for item actions on top of the page, too.

[em-jones]

I have a forked implementation that uses the render_resource_slot/3 implementation that just adds the item to the assigns before calling it. Would love to help us figure out a more long-term API for this