Layout and PiP improvements, new settings

- player is now a separate window on macOS
- add setting to disable pause when player is closed (fixes #40)
- add PiP settings:
  * Close PiP when starting playing other video
  * Close PiP when player is opened
  * Close PiP and open player when application
    enters foreground (iOS/tvOS) (fixes #37)
- new player placeholder when in PiP, context menu with exit option

macOS/InstancesSettings.swift

@@ -94,7 +94,7 @@ struct InstancesSettings: View {
                     }
                     .labelsHidden()
 
-                Text("Used to create links from videos, channels and playlist")
+                Text("Used to create links from videos, channels and playlists")
                     .font(.caption)
                     .foregroundColor(.secondary)
             }