Editor - "Back" button testing

[alexwarren]

I left some notes in #122 about some further testing that should be done with the new "Back" button in the editor:

• Test how this works in combination with the "turn counting" feature (e.g. with passages called [@2] and so on)
• This changes how clearing the screen works, by adding a "clear stack" where the screen contents are sent, so we can restore them. If games are able to clear the screen via JS we'll need to see how well this works with that.
• Also need to test how well this works with animated transitions.

In fact there's definitely a bug introduced by this with turn counting, as there's now a border separating the "turn" passage from the regular one. This should work the same as embedding, as if the turn passage's text were just part of the regular one's text - no border between them.

[RoperNo1]

I think you could add to this the state of a user input. If you go back it is not editable, you need to go back an additional section.