GTK 3 header bar

Property definitions

linux :: ui $ App :: native_header_bar
	# GTK 3 header bar
	var native_header_bar: GtkHeaderBar is lazy do
		var bar = new GtkHeaderBar
		bar.title = "app.nit" # TODO offer a portable API to name windows
		bar.show_close_button = true

		bar.add back_button.native

		return bar
	end
lib/linux/ui.nit:40,2--49,4