SequencePuzzleObject: Add sprite_modulate property

[wjt]

Most instances of the sequence puzzle use the same sprite for all the
objects, with their colour adjusted using modulate. But we modulate
the entire tree of nodes (rooted at a StaticBody2D) rather than just the
AnimatedSprite2D.

Back when the puzzle was first prototyped, modulate was applied just to
the AnimatedSprite2D, based on the pitch of the note. I removed this in
commit ffd20d0
("MusicalRock: Remove automatic modulation by note") which has this note:

This looks a bit weird in the editor because the rock's collision shapes
and "kick" interaction label also get modulated. But at runtime the
collision shapes are invisible, and the label is not actually used – it
just sets the position where the label belonging to the player will be
positioned. And I consider modulating the sprite to be a temporary
measure; it would be better to recolour the rocks.

The great thing about temporary measures is they tend not to be temporary!

I am working on replacing the interaction label (not used at runtime)
with an indicator floating above each interactable object, which is
visible at runtime, and so modulating the whole scene will no longer
suffice.

Add a sprite_modulate property which proxies through to the
AnimatedSprite2D within. Update every scene to use it. (Not all
sequence puzzles use this approach: special mention to After the Tremor
which has per-object sprites.)

In effect, this is a manual version of the approach I rejected in commit
ffd20d0:

(I tried using PropertyUtils to export the modulate property of the
AnimatedSprite2D. It solved this problem but in my opinion the cure was
worse than the disease: it added more complexity to what is otherwise a
very simple scene, and the resulting "Sprite Modulate" property in the
inspector had no documentation.)

If only Godot had a built-in way for a scene to expose properties of its
children without going all the way to editable children!

[github-actions]

Play this branch at https://play.threadbare.game/branches/endlessm/wjt/sequencepuzzleobject-add-sprite-modulate-property/.

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

[manuq]

Makes sense to use a proxy in this case. Ugh you had to do it in all StoryQuests!

[wjt]

It was quite an easy search-and-replace across the StoryQuests!

[wjt]

@jgbourque I presume that in at least 1 guide about the sequence puzzle, you show people how to adjust modulate to change the object's appearance. It is better now to use this new sprite_modulate property. modulate will still work for now, but assuming #2150 is completed it will also apply to the "interact with this" arrow in an ugly way. sprite_modulate does not have this property.