[RFC] Rename liip_imagine size filter

Hi guys, I want to rename our size filter from `sylius_16x16` to `sylius_icon`, `sylius_small`, `sylius_medium`, `sylius_large` etc, so we can change the size without renaming the filter.

This naming really bugging me.

```yml
liip_imagine:
    data_root: %kernel.root_dir%/../web/media/image
    filter_sets:
        sylius_16x16:
            filters:
                thumbnail: { size: [16, 16], mode: outbound }
        sylius_50x40:
            filters:
                thumbnail: { size: [50, 40], mode: outbound }
        sylius_90x60:
            filters:
                thumbnail: { size: [90, 60], mode: outbound }
        sylius_200x200:
            filters:
                thumbnail: { size: [200, 200], mode: outbound }

        sylius_262x255:
            filters:
                thumbnail: { size: [265, 255], mode: outbound }
        sylius_310x300:
            filters:
                thumbnail: { size: [310, 300], mode: outbound }
        sylius_610x600:
            filters:
                thumbnail: { size: [610, 600], mode: outbound }
        sylius_gallery:
            filters:
                thumbnail: { size: [640, 480], mode: outbound }
```
https://github.com/Sylius/Sylius/blob/master/app/config/config.yml#L135-L159

We do have a lot of sizes, should we remove some of them too? if so how many should we keep?