Powerpoint writer: handle speaker notes earlier in the conversion process

Internal change: We were getting bad results with the empty text box
created by the conversion of notes into an empty paragraph. So we now
convert the notes at the `blocksToSlide` stage, by `walkM`ing a
function over the blocks that returns `()`, and then filters the notes
out. This avoids the need to have a return value for speaker notes,
and thus avoids the empty TextBox.

Together with the previous commit (256f14c5a), closes #4477.

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