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/Playlists/PlaylistsView.swift

@@ -144,7 +144,7 @@ struct PlaylistsView: View {
 
                 Button {
                     player.playAll(items.compactMap(\.video))
-                    player.presentPlayer()
+                    player.show()
                 } label: {
                     HStack(spacing: 15) {
                         Image(systemName: "play.fill")