Proposal: make "hardware_id_wacom" more generic

[ferreiradaselva]

It feels a bit off that a vendor-specific event is exposed to the client.

Why not something like:

```
void (*hardware_specific_id)(void *data,
        struct zwp_tablet_tool_vX *zwp_tablet_tool_vX,
        uint32_t length,
        uint8_t *id);
```

The tablet name is already exposed on the event zwp_tablet_v2_listener.name. Using the name provided, the client could decide if would care or not about the specific model.

So, in the case o Wacom, the `length` would be `8` and the current `hardware_id_hi` and `hardware_id_lo` would be passed to `*model`.

This way, this event could be used for any other vendor (and if they don't have such id, the `length` would be `0`) and the client-side wouldn't contain vendor-specific API.

-Felipe