How to use visual effect view in NSWindow

Issue #610

Set NSVisualEffectView as contentView of NSWindow, and our main view as subview of it. Remember to set frame or autoresizing mask as non-direct content view does not get full size as the window

let mainView = MainView()
    .environment(\.managedObjectContext, coreDataManager.container.viewContext)

window = NSWindow(
    contentRect: .zero,
    styleMask: [.fullSizeContentView],
    backing: .buffered,
    defer: false
)
window.titlebarAppearsTransparent = true
window.center()
window.level = .statusBar
window.setFrameAutosaveName("MyApp")

let visualEffect = NSVisualEffectView()
visualEffect.blendingMode = .behindWindow
visualEffect.state = .active
visualEffect.material = .sidebar

let hosting = NSHostingView(rootView: mainView)
window.contentView = visualEffect
visualEffect.addSubview(hosting)
hosting.autoresizingMask = [.width, .height]