refactor naming and workflow of filtered images generation

[dbu]

i am tempted to cleanup some of the process and also naming how the filtered image files are created. especially with webp support, things are a bit muddy to me.

"resolve cache" is used to populate the cache. that is a strange name. we should streamline naming.

we have several classes that are not super obvious, with FilterPathContainer and FilterService.

WarmupCacheHandler calls FilterService::warmupCache but then also FilterService::getUrlOfFilteredImage (because of #1347). the only thing i see that the additional call does is triggering resolve events. i think it would be better to have warmupCache also trigger those events. ResolveCacheCommand does only call warmupCache, so it won't trigger the events if i understand correctly.