Open Radar

 
Summary:
I already opened rdar://46998451, but I think the use-case of "only wanting a path you can write to" such a common use-case that it should be handled by `NSOpenPanel`.

My use-case: I use `NSOpenPanel` to let the user select a directory where my app can output some files. I want to inform the user early, while the open panel is still shown, if the user tries to select a read-only directory, as I wouldn't be able to write to that.

Even if rdar://46998451 is fixed, I still think this feature has merits on its own.

Steps to Reproduce:
I have this code to open the panel:

```swift
@NSApplicationMain
final class AppDelegate: NSObject, NSApplicationDelegate {
	func convert() {
		let panel = NSOpenPanel()
		panel.delegate = self
		panel.canChooseFiles = false
		panel.canChooseDirectories = true
		panel.canCreateDirectories = true
		panel.allowsMultipleSelection = false
		panel.prompt = "Convert"
		panel.message = "Choose where to save the converted images"

		panel.beginSheetModal(for: window) {
			if $0 == .OK {
				self.startConversion(urls, outputDirectory: panel.urls.first!)
			}
		}
	}
}
```

Expected Results:
I expected to be able to set a property, let's call it `panel.allowReadOnlyPaths = false`, to make NSOpenPanel show an error if the user tries to select a read-only file or directory.

Actual Results:
Because of rdar://46998451, I can't even implement this manually, so I have to end up with a user unfriendly workaround that only shows an error after the open panel is dismissed, and then I have to show the open panel again.

Version:
macOS 10.14.2

Notes: