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

Shared/Videos/VideoCell.swift

@@ -31,7 +31,7 @@ struct VideoCell: View {
                 if inNavigationView {
                     player.playerNavigationLinkActive = true
                 } else {
-                    player.presentPlayer()
+                    player.show()
                 }
             }) {
                 content