Add a warning to the deletion prompts

If you want to delete an Asset, the prompt should be:

"Really delete asset? It might still be in use and deleting it can corrupt your content!"

If you want to delete a Timeline, the prompt should be:

"Really delete Timeline? You will loose all annotations on the Timeline!"

If you want to delete a Video on a Timeline, the prompt should be:

"Really remove video from Timeline? Annotations referring to this video might lose their point of reference!"