Check Permissions can open duplicate permissions dialogs

[ryanwaite]

Steps to reproduce

1. Click the ZoomIt menu in the menu bar
2. Select Check Permissions
3. Click Camera Settings — the macOS Settings dialog appears
4. Close the macOS Settings dialog (the ZoomIt permissions dialog is no longer visible)
5. Click the ZoomIt menu again and select Check Permissions

Result

Two ZoomIt permissions dialogs are now open.

Expected

Only one ZoomIt permissions dialog should ever be visible. Re-invoking Check Permissions should focus the existing dialog rather than open a second one.