Don't request manifests in sequence at init

[jfrer]

In GitLab by @paulpestov on Sep 21, 2023, 11:42

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 If there is no image pagination does not work #397)
• Check header
• Check metadata