Typing/Enter/Backspace should insert a paragraph if the magic block button was active

[oleq]

This should only happen when the block is already active, for instance, when a user activates it using the keyboard.

The paragraph would land in the model and be broadcasted to collaboration, track changes, etc.

P.S. Let's not forget about scrolling the viewport when the paragraph is inserted.

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A piece of #407.

[Reinmar]

Let's work on the typing at the end so if it turns out to be tricky, let's extract it to a separate ticket.