geom_text, geom_label: 'label' aesthetic shouldn't produce grouping

Example:

```python
data = {
'x': [0, 0, 0, 0],
'y': [1, 1, 1, 1],
'l': ["a", "b", "c", "d"],
}

ggplot(data, aes('x', 'y')) + \
geom_point(position=position_stack(mode='groups'), size=15) + \
geom_label(aes(label='l'), position=position_stack(mode='groups'))
```

It means that `aes(label='l')` adds additional implicit grouping to labels. But it shouldn't.

Expected behaviour: