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/Player.swift

@@ -4,7 +4,7 @@ import SwiftUI
 struct Player: NSViewControllerRepresentable {
     @EnvironmentObject<PlayerModel> private var player
 
-    var controller: PlayerViewController?
+    @State private var controller: PlayerViewController?
 
     init(controller: PlayerViewController? = nil) {
         self.controller = controller