Faster method for updating attributes; fix icons.

For updating layer attributes, we increase performance, by far, substituting something like:
```
zone_feat['count'] = points_count
self.aggregation_layer.updateFeature(zone_feat)
```
with something like:
```
count_index = self.aggregation_layer.fieldNameIndex('count')
fid = zone_feat.id()
self.aggregation_layer.changeAttributeValue(fid, count_index, int(points_count))
```
This pull request also fixes icon management.