Bug 31607: App menu items stop working on macOS

Avoid re-creating the hidden window, since this causes the nsMenuBarX
object that is associated with the app menu to be freed (which in
turn causes all of the app menu items to stop working).

More detail: There should only be one hidden window.
XREMain::XRE_mainRun() contains an explicit call to create the
hidden window and that is the normal path by which it is created.
However, when Tor Launcher's wizard/progress window is opened during
startup, a hidden window is created earlier as a side effect of
calls to nsAppShellService::GetHiddenWindow(). Then, when
XREMain::XRE_mainRun() creates its hidden window, the original one
is freed which also causes the app menu's nsMenuBarX object which
is associated with that window to be destroyed. When that happens,
the menuGroupOwner property within each Cocoa menu items's MenuItemInfo
object is cleared. This breaks the link that is necessary for
NativeMenuItemTarget's menuItemHit method to dispatch a menu item
event.