This fixes an issue where the editor of VocAlign Project 5 would fail to
open in the AudioPluginHost.

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.

Hosts such as REAPER normalise the program parameter value by dividing
the program value by the step count, rather than going via the
parameter's toNormalized function. To be compatible, we should use the
same scaling technique. At time of writing, the coversion process is
detailed under the heading "Conversion of normalized values" on this
page:

https://developer.steinberg.help/display/VST/Parameters+and+Automation

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

Previously, Xcode's main thread checker would complain when selecting
the "OpenGL Renderer" in the DemoRunner on macOS.