Add add tab button in TabContainer

[me2beats]

Describe the project you are working on

plugins

Describe the problem or limitation you are having in your project

I need Add tab "+" button in TabContainer like Godot Editor scene tabs have:

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Adding add tab button in TabContainer would solve the problem

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

1. new propery:
   bool show_add_tab -> true to show add tab button, false to hide it

2. new signal:
   int add_button_pressed
   This button should do nothing, only emit the signal.
   User should connect to the signal and add a child control with add_child() when the signal is fired/triggered.

If this enhancement will not be used often, can it be worked around with a few lines of script?

It seems this is doable using Tabs, but isn't trivial if you want it to work like in Godot scene tabs.

Is there a reason why this should be core and not an add-on in the asset library?

Add-a-tab button is a common UX solution for adding tabs. A lot of apps use it.

[YuriSizov]

A duplicate of a more general proposal #2250