Add findById overload for properties

[eth1platinum]

Allows lookup of properties directly like so:

    ThingSetReadOnlyProperty<float *> *imaxCellsProp = nullptr;
    if (ThingSetRegistry::findById(TS_ID_MODIINMAX, &imaxCellsProp) ...

[garethpotter]

As discussed, I don't think we can actually merge this, because the cast is not safe: you can look up a ThingSetProperty<float *> but provide a ThingSetProperty<uint8_t *> as the out value and it will happily comply.