Add support for bringing window to front (activating window).

[wilsonowilson]

At the moment, there is no way to bring the application window to the front of the screen or activate the window using this plugin. To my knowledge (at least on macOS), calling setWindowSize asynchronously may cause the app to crash due to NSApplication.shared.mainWindow being nil when the window is out of focus.

I've already implemented this for macOS in a separate plugin, but it would be very fitting to add support for all the platforms here in one unified plugin.