[PeterC89]

This PR is opened on behalf of myself.
All the code is AI generated but has been reviewed and functionally checked by a human.

I had a need to test some MOS plugins with an automation system and figured the rundown editor wasn't a bad place to do it.

What this does
You can now create and edit pieces using an external MOS plugin, such as a graphics or clip picker. The plugin opens inside the editor. When you save in the plugin, the piece is created or updated.

How it works

• Settings: add a plugin by giving it a name and a URL.
• Creating a piece: in a part's pieces list, click "Add from ". The plugin opens in a popup. When it sends back an item, a piece is created from it.
• Editing a piece: on a piece made by a plugin, click "Edit in ". The plugin opens with the piece's saved item, and its changes are saved to the piece straight away.

Things worth knowing

• Times: an item's start and duration use the item's own time base (objTB) if it has one. Otherwise they use the frame rate from Settings.
• Slow or missing plugins: if a plugin doesn't respond within 10 seconds, you get a warning with Retry and "Open in new tab".
• Saving: if a second item arrives while one is still saving, it is refused. A save that takes more than 30 seconds shows a timeout error.
• Edit failures: if saving an edit fails, the popup stays open and shows the error.
• Messages: the editor only accepts messages from the plugin's own frame and URL.

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