Make tutorial hint messages persistent

[CelticMinstrel]

Based on @stevecotton's #6615, this moves many of the tutorial hint messages into persistent overlay text. I played through the tutorial with these changes and I think it seems pretty good, but it probably needs a little refinement.

[CelticMinstrel]

Here's an example of what it looks like.

[Wedge009]

I haven't tried this but as with #6615 in principle it sounds like a good idea. It probably addresses the concerns mentioned in #6614 where the instructional part of the dialogue is missed amongst the 'flavour' text that some players may skip over.

[stevecotton]

Some of the message timings should change - for example, S01's caption="Recruiting" message currently displays when you move to the keep, but would fit better if it appeared at the start of the lesson, when the "I'll recruit some elves" message appears.

Some text is looking a bit cut-off (this was in a 1280x1024 Wesnoth window), only noticed it on this message.

[Pentarctagon]

So aside from the width issue, this seems good to merge? The message timings seem unrelated to this PR's purpose, since it's changing how a message is displayed rather than when.

[CelticMinstrel]

Yes, I don't know of anything to block merging this. The width issue would be an issue in the API, not in this change. It would be nice to get a review from someone other than stevecotton though.

[Pentarctagon]

@CelticMinstrel So should I click merge, or were you planning to?

[CelticMinstrel]

Sure, I can do it.