[5.x]: Add a "Delete" action to entry type edit pages

[mmikkel]

On field edit pages, there is a "Delete" action available in the "Save" button disclosure menu:

However, for entry types there is no "Delete" (or "Save and add another"):

In attempting to clean up (i.e. delete) unused entry types, edit pages are a good place to gauge whether or not a specific entry type is in use anywhere. To that end, it'd be great if it was possible to delete the entry type from this view, instead of having to go back to the entry type list page, find the correct, unused type in the table and delete it there.

[brandonkelly]

Added for 5.2! (288f186)

[brandonkelly]

5.2.0-beta.1 is out with that change.