WIP: Store which threads have been collected

[wjt]

Stacked on:

• Store InventoryItems as separate resource files #2689

Helps #2280

[wjt]

How to test:

1. New game.
2. Skip tutorial.
3. Go to Dev Archipelago
4. Talk to the first elder. i've patched him to show the level design quests
5. Chose the "Optional Threads" quest
6. On your first playthrough, collect only the green thread on the right. Don't try to avoid the guards.
7. Go back to Fray's End. Deposit the threads at the Loom. Watch the animation.
8. Quit to title then continue (works around a bug in this branch)
9. Go to Dev Archipelago again, and play the quest again. You should see that the green thread has a dither effect on it.
10. This time, avoid the guards and collect the red thread. Then deliberately allow the guards to catch you.
11. When you respawn, the red thread should still be gone.
12. Go back past the guards and collect the (dithered) green thread.
13. Take the 1 new thread you have collected to the loom.
14. Repeat step 8
15. Repeat step 9. You should see that the red thread is missing, and the green thread is stippled.

[github-actions]

Play this branch at https://play.threadbare.game/branches/endlessm/wjt/store-collected-threads/.

(This launches the game from the start, not directly at the change(s) in this pull request.)

[wjt]

This is a work in progress. I am more interested in review of the gameplay than of the code. The incompatible requirements I am trying to square are:

1. We want quests to be replayable
2. We want to be able to have optional threads in quests
3. You shouldn't be able to collect the same thread more than once (because you'll be able to spend your threads on powerups and otherwise you could just grind an easy quest and unlock the whole tech tree)...
4. ...but collectible threads are how we link scenes together.

In this branch, the behaviour of a previously-collected thread depends on whether the thread has a "next scene" configured:

• If it does, then it is shown in-world, but stippled, and collecting it again does not readd it to the inventory
• If it does not, then it is deleted immediately

The intuition is that optional threads should not link to another scene, while mandatory threads always do. (However I can already imagine cases where we might want to teleport the player after an optional thread...)