[UNREAL] Implicit Content Manifest and Content Window

[PedroRauizBeamable]

We are reworking the Content Workflow in Unreal to solve the following problems of the "DataTable is a Manifest" approach.

• Version Control is a bit annoying as the DataTable is a single serialized binary asset.
• The concept of a Local Manifest that is not just derived from the contents of the .beamable/content folder has proven unnecessary.

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Definition of Done

• A new Content Window with flows for Create/Rename/Delete and Editing individual content pieces.
  • Displays the content type clearly.
  • Displays the content status clearly (Added/Deleted/Modified/Up-to-Date)
  • Supports undo when editing each content object properties.
• Infers the current state of content from the state of the .beamable/content folder through the CLIs local-manifest command.
• Exposes functionality to Download (reset contents to what is in the realm) and Publish (push content to the currently selected realm).
• Looks like the Unreal Content Browser and enables project-customization of the visual representation of each piece of content in the window.