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.

This change is designed to fix issues with views stealing focus and
being brought to the front when popup menus are dismissed.

To test this behaviour:
- Open two instances "a" and "b" of a plugin editor containing a
  ComboBox
- Click the ComboBox in editor "a", opening its PopupMenu
- Drag the titlebar of editor "b" to move it
- The PopupMenu should be dismissed, but editor "a" should *not* steal
  keyboard focus or be brought to the front