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

gdk/gdkevents.h

@@ -824,6 +824,7 @@ struct _GdkEventScroll
   gdouble x_root, y_root;
   gdouble delta_x;
   gdouble delta_y;
+  guint is_stop : 1;
 };
 
 /**
@@ -1377,6 +1378,9 @@ gboolean  gdk_event_get_scroll_deltas   (const GdkEvent *event,
                                          gdouble         *delta_x,
                                          gdouble         *delta_y);
 
+GDK_AVAILABLE_IN_3_20
+gboolean  gdk_event_is_scroll_stop_event (const GdkEvent *event);
+
 GDK_AVAILABLE_IN_ALL
 gboolean  gdk_event_get_axis            (const GdkEvent  *event,
                                          GdkAxisUse       axis_use,