Allow nullable buttons for tools

[thijskuilman]

I find that you sometimes do not want to display a button when creating a custom extension. For example, today I created a custom extension that adds a title block at the start, similarly how Confluence does it:

In this case, I do not want my custom extension to have a button in the toolbar, since my custom functionality is all in the background. That is why I made the button attribute nullable in this PR.