Azayaka only updates monitor count on startup

[lihaoyun6]

I have an iPad as a sidecar display, and Azayaka can correctly identify it.
But when I disconnected the iPad from the Mac, Azayaka still showed two displays in menu.

I saw the following code in Menu.swift:

func menuWillOpen(_ menu: NSMenu) {
    if streamType == nil { // not recording
     -> updateAvailableContent(buildMenu: false)
        updateMenu()
    }
}

Why not use updateAvailableContent(buildMenu: true)?

[Mnpn]

Hi! Following up on the conversation in #14 - this should be fixed as of 9780acf by observing display device changes. Thanks!