eventcontrollerscroll: Conditionally propagate ::scroll

eventcontrollerscroll: Conditionally propagate ::scroll

Currently, gtk_event_controller_scroll_handle_event() always returns
TRUE if it is handled, which stops the propagation of the event. If
there’s a single GtkEventControllerScroll in the widget hierarchy, that
means that no others will run, depending on the propagation phase. In
Nautilus, this can be observed when adding a scroll controller to the
GtkScrolledWindow (ctrl-scrolling controls the zoom level) - either the
scrolling or the zooming breaks.

Fixes https://gitlab.gnome.org/GNOME/gtk/issues/45