Need to have hook point to allow "on demand" styling of items

[kikonen]

Should way of defining styling of items using config, via function.

i.e. function, which can determine dynamically what classes to apply to item.

Does not need to be fully dynamic (i.e. only support running it at item build time) for performance reasons (calling functions from render would be slow and cause that svelte cannot optimize render logic well).

[kikonen]

Will, for now, rely into item.item_class for this. I.e. not dynamic, but caller can setup styles at point of time when fetch is done.