Add dynamic image size / Thumb Index about froshwebp

[JoshuaBehrens]

btw. there is the prev and nex - nav without image webp atm.

Theme files are not part of this plugin. Only media manager managed media are currently supported.

[Hotte512]

may be i did not describe well what i ment with prev/next picture, it is not a theme file, these are Product images as well.

And I thought about adding settings for media managers thumb index options into the Plugin-config, so someone could tell webp which Index to use on detail for image.tpl and images.tpl and for listing product bos etc.

[shyim]

I have added that with prev and next to #33
If you want dynamic image sizes look at https://github.com/FriendsOfShopware/FroshTemplateExtensions