Don't request manifests in sequence at init

For various features we currently request all manifests with a collection at init time. This is not a scalable apporoach since there can be many sequence items in a collection. So we need to make sure that we retrieve the same information with other methods.

• One issue was already resolved by adding the label key to sequence items so the TreeView doesn't need the list of manifests anymore. (See #397 (closed))
• Check header
• Check metadata