Implement Menu > Import and associated command

[teh-cmc]

The last missing piece in the "open data in-place" story.

It does what you think it does: Menu > Import is like Menu > Open, but imports the data in the current recording rather than create a new one.

• DNM: requires Implement support for in-place drag-n-drop #7880

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[teh-cmc]

I interpret that silence as "no objections" 🙃