Hide the icon when an app has no visible windows, or when all of its
visible windows would have no taskbar entry (i.e., they are owned or
have WS_EX_TOOLWINDOW or WS_EX_NOACTIVATE, but not WS_EX_APPWINDOW).
The dock icon returns if those conditions are no longer satisfied.

This behavior is behind a Mac Driver registry key,
EagerDockIconHiding (defaulting to true), to allow toggling it on a
per-app or global basis.