Allow buttons for all destination bundles

Problem/Motivation

Currently, when the entity reference allows for more than one target bundle, this module's widget creates a dropdown to select what bundle to use when creating an entity to reference:

Particularly if there are only a small number of choices available, it would be better UX to have separate buttons instead. In the example above, it would be better to have buttons labelled "Add new link" and "Add new location".

Steps to reproduce

On a content type, create an entity reference field that allows for two or more target bundles. Use a complex IEF widget for the form display.

Proposed resolution

Allow a site builder to have multiple buttons instead of a dropdown. Perhaps it would be easier if this was a configuration option on the current widget, but creating a new widget would be another possibility to consider.