[REQUEST] Custom UI elements

Please add an API for allowing custom field UI elements.

For example:
When adding a “Post Object” field, we currently get a SELECT list. It would be great to be able to create different input elements (such as checkboxes, or a fancy combo box w/ drag-n-drop ordering).

Ideally, such an API would have both a LOAD hook (for passing default values into the widget) and a SAVE hook (for processing the values and passing data in a consistent manner back to ACF).