Merge: app.nit on GNU/Linux: implement multiple windows support using a GtkStack

This PR implement support for mobile-like multiple windows on GNU/Linux with GTK. It uses a single GTK window and a GtkStack showing one app.nit window at a time. This results in a single window with changing content.

The change to ListView provides a scrollable list of items, which is closer to the mobile equivalent.

Support for a back button will be introduced in a future PR.

Pull-Request: #1990
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc lib/gtk/v3_4/gtk_widgets_ext.nit
Simple merge

diff --cc lib/linux/ui.nit
Simple merge