add auto-completion for narration

[dtrai2]

third time's the charm

I accidentally deleted my changes again and closed the pull request #1816, luckily we have something like git reflog with which I could reconstruct it.

Anyways this is still my attempt to add the auto-completion feature for the narration text field. It is still not perfect, but I didn't want to let the changes go to waste. Maybe it's good enough like this or someone might help out to finish it (see my last comment on #1816).

closes #1535, #1145