Implement some kind of presenter mode

[jevakallio]

Currently you need to remember what you typed, and what is coming next. This is suboptimal.

It would be cool if we could implement some kind of presenter view, that would show the entire file as it will look like e.g. faded out like a comment, and the next part that the user is going to type. This is going to be a bear to implement.

Alternatively, we coud allow the user to annotate code blocks (delimited by soft stops) to remind them what is coming next. When entering the next block, we could show the presenter a hint of what is coming. This would be easier, but is it useful?

Both of these would involve dealing with windowing, e.g. having two windows with a shared buffer but different contents.