wayland: add gdk_event_is_scroll_stop_event()

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