Root GtkStack used to simulate the many app.nit windows

Property definitions

linux :: ui $ App :: native_stack
	# Root `GtkStack` used to simulate the many app.nit windows
	var native_stack: GtkStack is lazy do
		var stack = new GtkStack
		stack.homogeneous = false
		return stack
	end
lib/linux/ui.nit:51,2--56,4