Tabs added to existing tabset do not have prepopulated content

Problem/Motivation

When first adding the tab widget the default tabs are populated with a sample paragraph. You can expand the available tabs by clicking the button again and increasing the tab count. However, this does not populate the tabs with a sample paragraph. This makes editing them difficult or impossible.

Steps to reproduce

1. Ensure you have a text format configured with this plugin
2. Open a node add/edit page
3. Click the button to add a tab set
4. Select your tab set and click the button again, increasing the number of tabs
5. You should see that new tabs are added to reach the new count you set
6. Select one of these new tabs - you will see it has no content.
7. Try clicking inside the tab, you will see you cannot easily select the content to edit it

Proposed resolution

New tabs should be populated with sample content.

Remaining tasks

I will attach an MR and patch shortly.