Add HdyPaginator

Some notes:

Yes, it reallocates children on every frame of animation. It would be a lot more efficient to have a GdkWindow and move it. But tis causes weird stuttering either just with touch input or specifically in phosh.

```
(handy-0.0-demo:2): Gtk-CRITICAL **: 21:23:59.342: gtk_widget_draw: assertion '!widget->priv->alloc_needed_on_child' failed
```

~~~Interestingly, this doesn't happen with GtkStack, only with HdyLeaflet.~~~