Dragging destination support

[barnettben]

It looks like STTextView on macOS doesn't yet support dragging text in from other apps.

I'd be happy to take a crack at adding this.

I see there are stubs/files where dragging destination code would go. It also looks at first glance like readSelection(from: NSPasteboard, type: NSPasteboard.PasteboardType) -> Bool would do the job of reading in dragging pasteboard content.

I was thinking just text (plain/rtf) for now, not dragging files in. That way no changes would be needed to the pasteboard reading code.

For moving the insertion point while dragging, I've not looked closely at the mouse tracking code yet, but would seek to reuse anything existing before writing new code.

Would this be something useful?

[krzyzanowskim]

totally. that is indeed an overlook

[barnettben]

Cool, thanks, I'll have a go at it then.