V15: Bugfix: Add "Not Found" empty state to detail workspaces

[madsrasmussen]

This pull request introduces a "Not Found" view that will be displayed when an entity is not in the detail store. This state occurs in several scenarios:

• When you navigate to a workspace with a unique that can't be found on the server.
• When you delete the same entity from the menu while already in the workspace
• When any other process deletes the entity (signal r etc.)

The core functionality is built into the "UmbEntityDetailWorkspaceBase," which subscribes to changes in the detail store. If an entity is removed from the store, the data will be cleared in the workspace context, and a not found view will be displayed.

I have added a couple of additional features to complete this feature:

• A loading state for when the entity data is being scaffolded or loaded.
• A new "umb-entity-detail-workspace-editor" element that implements the not-found view + renders the workspace entity action at the right time (only when an entity is fully created).

This feature is currently implemented in the following workspaces. However, we still need to align some of the remaining workspaces to ensure it work in all cases:

• Data Type
• Language
• Webhook
• Dictionary
• Template
• Script
• Partial View
• User
• User Group
• Member Group