Add support for window minimization

[GeertJohan]

When the flutter application signals to pop the whole application, we should probably iconify or minimize the application..

SystemNavigator.pop method on flutter/plaform channel:
https://github.com/flutter/engine/blob/master/shell/platform/android/io/flutter/embedding/engine/systemchannels/PlatformChannel.java#L113

Should be routed to GLFW hide/show or iconify:
https://godoc.org/github.com/go-gl/glfw/v3.2/glfw#Window.Hide
https://godoc.org/github.com/go-gl/glfw/v3.2/glfw#Window.Show
https://godoc.org/github.com/go-gl/glfw/v3.2/glfw#Window.Iconify

And

Callback on focus or iconify changes:
https://godoc.org/github.com/go-gl/glfw/v3.2/glfw#Window.SetFocusCallback
https://godoc.org/github.com/go-gl/glfw/v3.2/glfw#Window.SetIconifyCallback

Should be routed to LifecycleChannel:
https://github.com/flutter/engine/blob/master/shell/platform/android/io/flutter/embedding/engine/systemchannels/LifecycleChannel.java

[GeertJohan]

This could be part of a bigger effort to separate GLFW as a renderer, which would then implement platform channels.

[GeertJohan]

Starting work on this now.