Make MeshLibrary use PropertyListHelper

[YeldhamDev]

This PR ports from the old way MeshLibrary uses to show (common properties in the inspector) and manipulate (options inside a dropdown in the toolbar) its data, to instead using PropertyListHelper (already used in stuff like PopupMenu, TileMap, etc). Allowing to easily create, delete, and move items.

[KoBeWi]

Uh... #115281
(I should've mentioned PropertyListHelper somewhere I guess)

[YeldhamDev]

Well, shit...

But hey, due to mine being more recent, there are no file conflicts at least... 🫠

EDIT: Testing your version, I actually can't add nor remove elements, and the dropdown options for that are still present.

[YeldhamDev]

Alright, good news: it can now handle the gaps between indexes.
Bad news: It does by filling the gaps with empty items. Not ideal.

It's in a state where it could be merged, but the ideal solution would be that when a library with gaps is detected, it would be shrunk, and any GridMap that uses it would be updated with the new indexes.

[KoBeWi]

When deleting an element, all elements after it will be shifted, causing the GridMaps to become scrambled. I suppose this is the reason why MeshLibrary is using a sparse array. Your PR ignores that completely, my PR tries to keep this characteristic, albeit with limited results.

tbh the MeshLibrary as it is now reminds me of the old TileSet. Each tile is a separate entity in a very loose list. The main problem is that there is no proper editor for MeshLibrary at all and editing the properties in the inspector is a workaround, and not a very friendly one. The MeshLibrary properties should not be in the inspector at all, but right now there is no better way to edit them.

That said, if we ever want to add a dedicated editor, adding PropertyListHelper does not conflict with it (see #92472), so this PR should be fine (if we accept GridMap scrambling xd). It's also more functional than my one.

[KoBeWi]

This should be probably changed to LocalVector, since the structure can no longer have holes.

[KoBeWi]

This is technically unsafe. If the missing index is a hole, the further items will get erased if they were already assigned. Unlikely to happen with normal usage though.

[YeldhamDev]

I've been testing this PR with this project: https://github.com/Nodragem/top-down-action-adventure-starter-kit

It has a library with separate gaps across it, and it didn't result in any data loss while manipulating it.

[KoBeWi]

Yeah, they'd need to be loaded out of order, which can only happen if you manually reorder them or due to some bug.

[YeldhamDev]

I introduced a little hack to avoid polluting the GridMap editor: items that are created to fill gaps have a new property called hide_from_list set to true. As the name implies, it hides those items from get_item_list(), which is what GridMap editor uses.

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The main problem is that there is no proper editor for MeshLibrary at all and editing the properties in the inspector is a workaround, and not a very friendly one. The MeshLibrary properties should not be in the inspector at all, but right now there is no better way to edit them.

You bring a good point. Maybe the way to go would simply be creating a proper editor for MeshLibrary. It wouldn't need to be too complex, just a grid of items to the left, and an inspector for them to the right, with a toolbar at the top for item creation/deletion.