Preview an operator's result when hovering it on the canvas #9057

Feature Summary

To see what an operator produced, a user has to select it and open the result panel, even for a quick look at a few rows while building a workflow.

Part of #9052.

Proposed Solution or Design

As settled in #8842: resting the pointer on an operator for a moment shows a small card beside it with its first rows, its error, or how to keep a result the run did not keep. The card should not open while the pointer only passes over, drags, or clicks, and the preview can be turned off under Layers.

Affected Area

Workflow UI