How to show context popover from SwiftUI for macOS

Issue #630

For SwiftUI app using NSPopover, to show context popover menu, we can ask for windows array, get the _NSPopoverWindow and calculate the position. Note that origin of macOS screen is bottom left

(lldb) po NSApp.windows
▿ 2 elements
  - 0 : <NSStatusBarWindow: 0x101a02700>
  - 1 : <_NSPopoverWindow: 0x101c01060>

let handler = MenuHandler()
handler.add(title: "About", action: onAbout)
handler.add(title: "Quit", action: onQuit)

guard let window = NSApp.windows.last else { return }
let position = CGPoint(
    x: window.frame.maxX - 100,
    y: window.frame.minY + 80
)
handler.menu.popUp(positioning: nil, at: position, in: nil)