Add "Delete" button to Manage Blocks page for Layouts

[stpaultim]

Description of the need

In issue #2894 we are creating a simple wizard to create a layout specific to one or more entity types. One criticism of the current PR is that a person my create and save a new layout before they get to the manage blocks page, at which point they might realize that don't want the layout at all and they have to go back to the Manage Layouts page to remove it.

This could be fixed by changing the way Layouts are saved so that a new layout is not created and saved until the person get's to the manage blocks page and saves it there. I believe that this would mirror how views work. One can work their way through the process of creating a view without saving it, until they get to the final step.

This feature is for an alternative, easier approach, to at least partially solving this problem. In many places in the Backdrop UI, we provide a delete button next to the save button, to make it easy for someone to remove content or configuration that they no longer need.

Proposed solution

Add this "Delete Layout" button to the Manage Blocks page for each layout.

Alternatives that have been considered

The alternative mentioned above is that we NOT save a layout until we get to the "Save Layout" button on the Manage Blocks page, to make it easier for someone to back out of a layout that they started, but decided they don't want to keep.

Additional information

We provide a "Delete" button on node edit pages.

We provide a "Delete" button on the configure page for File Types

We provide a "Delete" button on the configure tab for custom menus.