wayland: add gdk_event_is_scroll_stop_event()

And use it to handle kinetic scrolling in the GtkScrolledWindow.

However, dropping the delta check causes the X11-based kinetic
scroll to break since we don't have the stop event here. Correct handling of
xf86-input-libinput-based scroll events is still being discussed.

https://bugzilla.gnome.org/show_bug.cgi?id=756729

gtk/gtkscrolledwindow.c

@@ -3321,7 +3321,7 @@ gtk_scrolled_window_scroll_event (GtkWidget      *widget,
        * after scrolling finished, start kinetic scrolling when this
        * happens.
        */
-      if (delta_y == 0 && delta_x == 0)
+      if (gdk_event_is_scroll_stop_event ((GdkEvent *) event))
         {
           handled = TRUE;
           start_deceleration = TRUE;