Changes 15: More ContentStorageHandler class methods

[bastianallgeier]

This PR …

• 🚨 Merge first: Changes 1: Refactor content storage to use new VersionId class #6436
• 🚨 Merge first: Changes 2: Plain Text Handler Refactoring #6439
• 🚨 Merge first: Changes 3: New Version class #6442
• 🚨 Merge first: Changes 4: Refactor PlainTextContentStorageHandler to use Language object #6448
• 🚨 Merge first: Changes 5: ContentStorageHandler Abstract #6449
• 🚨 Merge first: Changes 6: Version class improvements and unit tests #6450
• 🚨 Merge first: Changes 7: Move ContentStorage methods to ContentStorageHandler #6454
• 🚨 Merge first: Changes 8: New ModelWithContent::version method #6455
• 🚨 Merge first: Changes 9: Remove content storage class #6456
• 🚨 Merge first: Changes 10: New MemoryContentStorageHandler #6457
• 🚨 Merge first: Changes 11: Language::ensure & Languages::ensure #6483
• 🚨 Merge first: Changes 12: More Version class improvements #6490
• 🚨 Merge first: Changes 13: New translation classes #6491
• 🚨 Merge first: Changes 14: Model prop fixes #6499

Reasoning

This PR is grouping together additional ContentStorageHandler changes that are needed in later steps. It just tries to keep future PRs more manageable and also helps to keep code coverage and unit tests under control.

Features

• New ContentStorageHandler::model method to access the parent model. We need this later when working with clones in model actions.
• New ContentStorageHandler::from method. This method will take an existing ContentStorageHandler instance and create a new instance by copying over the same versions. This is needed later when we convert a model from PlainTextContentStorageHandler to MemoryContentStorageHandler for example.

Breaking changes

None expected

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add changes & docs to release notes draft in Notion

[lukasbestle]

I'm still not quite sure how we can clean up the methods in ContentStorageHandler and this makes the interface idea harder, but let's see. Problem for future Basti and Lukas when everything else has settled :)