✨ Add insertParagraph buttons

[micgro42]

Exemplary implementation of the insertParagraph buttons.

The buttons need access to the current view to execute the command. Due to javascript scoping rules we have to provide a function to get the view when we need it instead of a reference to the view itself.

Also, strange things are happening if we create the buttons right in the schema node where we need them. Thus creating them in a local variable and adding them when needed.

@annda, @JayDeeDee Please especially point out any places where you'd feel that more comments would be helpful in the future.

ToDo:

• [ ] decide where we actually want to use them
• [ ] styling