Document > Clear Artboards should not delete its children

[Keavon]

Instead of deleting its children, it should ungroup those children from the artboard and then delete just the artboard, leaving the artwork behind. The code for ungrouping with Ctrl+Shift+G can probably be referenced to achieve a similar outcome here.

We also want the same for deleting an artboard from the Artboard tool, where the artwork remains behind. (However, when deleting from the Layers panel, we do want the current behavior of deleting its children since that's also how deleting folders in the Layers panel works.)

A continuation of #1502