Refactor confusing 'displayCount' into 'fetchAmount', this is what it actually is.
Change synchronize logic so it doesn't keep X messages per folder anymore. Keep everything and optionally fetch more older messages.
Remove visible limit information from LocalFolder. We have a ghost column a result for now.
Edge case of no local messages and no extra fetching.
Adapt the translations to reflect the true meaning.
Fix wrong message fetch count in intial fetch and remove unnecesary check.