Add the ability to customize the text of submit buttons

Problem/Motivation

In many cases site owners want to be able to customize the text of buttons in IEF, for example:

My customer has two issues with the way that inline entity forms labeling works:

1. Wherever "new" appears we need to remove that... so, instead of "Add new TA provider/consultant" it should say "Add TA provider/consultant". This happens both in the field and the button for submission
2. Instead of "Create" in the input buttons, it should say "Add"

In much the same way as we have custom labels (singular and plural), is it possible to override the button texts?

(example from the original posing of this issue)

Proposed resolution

In 'manage form display' when inline entity form is selected for a field provide an 'override submit button text' option.

Remaining tasks

1. Create patch
2. Review patch
3. Test patch

User interface changes

In 'manage form display' when a table row using IEF is expanded the last option in the list will be 'override submit button text' with a checkbox. When checked the behavior will follow the same pattern as when the 'override labels' option is checked, ie. fields to override the submit text will expand.

API changes

?

Data model changes

?