Material assets support: property widget, materials modal, generic asset commands

[vincentfretin]

Description

Port of aframevr#859 to the editor, adapted to the undo/redo command system. Companion to aframevr/aframe#5846 (materials as assets and property type).

Material property widget and Material Assets modal

• The material property type is rendered by a MaterialWidget: the raw reference string (#myMaterial or an inline material(...) definition) committed on blur like selector types, plus a swatch previewing the referenced material's color/texture.
• Clicking the swatch opens a Material Assets modal that lists every <a-material> with color/texture swatches, applies the selected asset to the property (USE SELECTED or double-click), edits the selected material live with schema-driven widgets (shader read-only), and creates new <a-material> assets. Map properties use the textures modal, stacked on top (Modal is now stacking-aware: only the topmost open modal reacts to ESC/outside clicks).
• When the material component's material property is set, the other (ignored) property rows are hidden in the component panel.

Command system integration

• New generic AssetCreateCommand (assetcreate): creates any asset element in <a-assets> (a-material, a-mixin, img, audio, video, a-asset-item, ...) with optional attributes; a unique <base>-N id is generated when none is given and stays stable across undo/redo. Optional callback receives the created element.
• New generic AssetUpdateCommand (assetupdate): updates an asset attribute with raw old/new attribute strings; consecutive edits of the same attribute merge in history like entity updates. Inline materials (no id) are referenced directly and keyed by their inline string.
• EntityUpdateCommand treats the material property type like selector types (raw reference string as old/new value).
• ModalMaterials executes commands and refreshes on assetcreate/assetremove/assetupdate events, so it reflects undo/redo while open; MaterialWidget repaints its swatch on asset updates.
• ModalTextures now creates new image assets through assetcreate (undoable) instead of the direct insertNewAsset helper, which is removed.
• New generic AssetRemoveCommand (assetremove): removes an asset element, capturing tag name, attributes and position so undo recreates it in place; for <a-material>, entities referencing the asset are re-resolved on undo so they pick up the recreated THREE.Material. UI: DELETE button in the materials modal (disabled for inline materials, confirmation mentions how many entities use the material) and a trash button on each asset image in the textures modal. assetupdate also supports renaming an asset id (attribute: 'id'), and the materials modal exposes an id field that renames the asset and updates every entity referencing it in one undoable multi command (payload referenced by id strings so it stays serializable).

Verified against an aframe build of the PR branch

• Widget commit produces one entityupdate (#gold → #crateMat); undo/redo toggles the shared material on the entity.
• Two rapid roughness edits in the modal merge into a single assetupdate (0.2 → 1); undo restores 0.2 on the material and attribute.
• NEW MATERIAL: undo removes the asset (modal refreshes), redo recreates it with the same id and reselects it.
• Texture pick from a material's src row goes through the stacked textures modal and lands as assetupdate src → #crateImg; undo clears both attribute and map.
• assetcreate also verified for img (undo/redo) and a-mixin with attributes.

Requires an A-Frame build with aframevr/aframe#5846 for the material property type; the command changes themselves are inert without it.

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