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/MenuCommands.swift

@@ -62,10 +62,18 @@ struct MenuCommands: Commands {
             .disabled(model.player?.queue.isEmpty ?? true)
             .keyboardShortcut("s")
 
-            Button((model.player?.presentingPlayer ?? true) ? "Hide Player" : "Show Player") {
+            Button(togglePlayerLabel) {
                 model.player?.togglePlayer()
             }
             .keyboardShortcut("o")
         }
     }
+
+    private var togglePlayerLabel: String {
+        #if os(macOS)
+            "Show Player"
+        #else
+            (model.player?.presentingPlayer ?? true) ? "Hide Player" : "Show Player"
+        #endif
+    }
 }