Make the title of the extension properties create button consistent with the title of it's toggle section

[rpkoller]

Is your feature request related to a problem? Please describe
Currently one of the toggle sections is called Extension properties while at the same time the title of it's plus button is Create new list item. The title isn't directly visible, nor a tooltip is showing up if you remain with the mouse cursor on hover, but from a usability perspective the inconsistent terminology makes me think as a user what does list item mean at all in that context?

Describe the solution you'd like

Why not simply use Create new extension property for the button title instead? That way the microcopy would be consistent for the toggle section?

Describe alternatives you've considered

Additional context

[barmac]

I think it makes sense though it would require a change in the group component API so that one can add custom plus button label.

[barmac]

What if it announced sth like "Create new list item - Extension properties"? Perhaps it could be easier to implement.