Add API to set progress for application, reflected in Dock/task bar

A common feature between macOS and Windows is to reflect the state of the application as a progress, e.g. on how much time there's left on a timer, or to download a file.

On Windows this was handled by the now defunct QWinTaskbarProgress. On macOS we do not have an API for this, but third party solutions exist:https://github.com/sindresorhus/DockProgress

It would be useful to have a cross platform API for this.