The 'wrapper' accessibility handler is now ignored if a menu item has a
custom component, and has no submenu, and cannot be triggered
automatically. This avoids the case where a custom menu item may end up
with a wrapper accessibility handler that has no useful actions.

This patch also adds a 'label' argument to the addCustomItem functions,
which allows text for the screen reader to be supplied in the case where
a custom component is in use, but the menu item has accessibility
actions.

Effects of this change for windows with native titlebars:
- Resizable windows without maximise buttons will now only allow the
  'zoom' behaviour from the rightmost titlebar button (i.e. this button
  will not allow entering full-screen mode)
- Non-resizable windows will grey-out the rightmost titlebar button,
  whether or not the maximise button is enabled

This works around a rare issue in the DialogsDemo where the AlertWindow
would fail to display because no peer had focus.

When FileChooser instances were created, launched, and hidden all inside
the same event callback on macOS 12.0.1, the chooser dialog sometimes
remained open. This could cause problems including crashes, as closing
the dialog would attempt to call a completion handler block referencing
an already-deleted FileChooser::Native instance.

Opening the chooser panel later on the message thread seems to resolve
the issue.

This mirrors the behaviour on Linux, where maximizing a window always
fills the screen, even if a constrainer is active.

If you don't want to allow your component to fill the screen, disable
the maximize button in the window's titlebar when creating the window
(i.e. ensure DocumentWindow::maximiseButton is not set in the required
buttons).