App resize listener

Hello there,

This release brings a way to be notified that that app window size changed within components. It can be done by implementing the Resizer interface:

type myCompo struct{
    app.Compo
}

func (c *myCompo) OnAppResize(ctx app.Context) {
    // Handle app resizing.
}

It can be quite a handful when building layout components. Shell and Flow has been refactored and now uses this mechanism.