Add new tab icon button at the end of DocTabs

[jaredtmartin]

Checklist

• [X] I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• [X] This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

I need a way to have a + icon the user can click on to open another document similar to Chrome tabs.

Is it possible to construct a solution with the existing API?

I tried putting the button and the Doctabs in a container, but that prevents the document content from filling the entire window. I'll have to create a custom widget that inherits from the DocTabs

Describe the solution you'd like to see.

It would be nice to have an optional button that uses the + icon which the user can click to open another document which would run an event handler similar to OnClosed but called something like OnNew.

[jaredtmartin]

Sorry I just realized I just have to set CreateTab. However in my defense, this is not in the docs yet. Thank you for your great work on this project.