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/OpenWindow.swift

@@ -0,0 +1,40 @@
+import AppKit
+import Foundation
+
+enum OpenWindow: String, CaseIterable {
+    case player, main
+
+    var window: NSWindow? {
+        // this is not solid but works as long as there is only two windows in the app
+        // needs to be changed in case we ever have more windows to handle
+
+        switch self {
+        case .player:
+            return NSApplication.shared.windows.last
+        case .main:
+            return NSApplication.shared.windows.first
+        }
+    }
+
+    func focus() {
+        window?.makeKeyAndOrderFront(self)
+    }
+
+    var location: String {
+        switch self {
+        case .player:
+            return rawValue
+        case .main:
+            return ""
+        }
+    }
+
+    func open() {
+        switch self {
+        case .player:
+            NSWorkspace.shared.open(URL(string: "yattee://player")!)
+        case .main:
+            Self.main.focus()
+        }
+    }
+}