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).

This change allows mouse events (including enter and exit events) to
reach unfocused views on macOS. This matches the behaviour of unfocused
windows on Linux and Windows, where components paint in their "hovered"
states even when the application window is in the background.

As a byproduct of using tracking areas on macOS, we can remove the fake
mouse move generator.

This new factoring is a bit more typesafe, as it avoids casting
internals to void*.

It also allows cursors to scale appropriately on high resolution
displays on Windows.