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

Model/Player/PlayerTVMenu.swift

@@ -66,7 +66,7 @@ extension PlayerModel {
 
     func rebuildTVMenu() {
         #if os(tvOS)
-            avPlayerViewController?.transportBarCustomMenuItems = [
+            controller?.playerView.transportBarCustomMenuItems = [
                 restoreLastSkippedSegmentAction,
                 rateMenu,
                 streamsMenu