Fix typo

Okay, remove previous request only if it has same parent as this one. Fixes the last note of previous commit (which was nearly impossible to hit, but whatever, just cleaning implementation)

- Only allow one engine instance running at the moment - When adding new map preview call to the queue, check if previous map preview request has started, and if no, remove it from queue. Could be harmful if you manage to request previews for two widgets in less time than needed to generate one.