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
element where we need them. Thus creating them in a local variable and
adding them when needed.

ToDo:
* decide where we actually want to use them
* styling

Why didn't they show up during `yarn watch` ? o.O

The previous implementation was flawed beyond repair, because there was
only ever a single instance of the buttons. They were in effect only
attached to the very last element of those which were supposed to have
them.

This new implementation lets ProseMirror create the buttons and executes
the event based on their class and get's the view from the global
ProseMirror field.