Description:

Advantages:

• Single-property components are inflexible in that you cannot add a new property to the schema without having it be a breaking change. I was looking at Alphatest addition #2297, and I couldn't add a new prop without a breaking change.
• Less confusing having to guess if a component is single-property or multi-property, for example, is it hand-controls="hand: left" or hand-controls="left" (some controllers are single-prop, some are multi)? Is it gltf-model="src: #model" or gltf-model="#model" (some loaders are single-prop, some are multi). It's been inconsistent so I've seen people make mistakes including myself
• Less documentation about schemas in the component API (https://aframe.io/docs/0.5.0/core/component.html#single-property-schema is a bit awkward to explain)
• Reduced API surface/branching in the component API (e.g., if you do this, then it's single prop, if you specify this, it's multi prop)
• Can delete a lot of code where we check isSingleProp in a lot of places in the core component code

If we do this, then we'd need to introduce a default property for cases such as position, rotation, scale that designates if the component value is used in single-prop mode:

schema: {
  position: {type: 'vec3', default: true}
}