AProperty: name suggestions

`AProperty` was introduced in 2025Q1 as a copy-paste of Qt's `QProperty`. `AProperty` is mostly used to combine a value and a signal that notifies about value changes. This main use-case is not reflected by `property` name. `property` is too generic.

Decision must be made prior to `v8.0.0`.

I'd like to suggest a better name for

```
AProperty
APropertyDef
APropertyPrecomputed
```

## reactive

```
AReactive
AReactiveDef
AReactivePrecomputed
```

## observable

```
AObservable
AObservableDef
AObservablePrecomputed
```