gesturesingle: Pass through scroll events

We want to have them in GtkGestureDrag/Pan/Swipe.

Ignore motion events when no button is pressed - otherwise moving the
widget under the poitner in response to scrolling or a touchpad swipe would
interfere with them.

FIXME: I don't remmeber what ther button == 0 is about.

gtk/gtkgesturesingle.c

@@ -196,6 +196,9 @@ gtk_gesture_single_handle_event (GtkEventController *controller,
             }
         }
 
+      if (button == 0)
+        return FALSE;
+
       break;
     case GDK_TOUCHPAD_HOLD:
       if (gdk_touchpad_event_get_n_fingers (event) == 1)
@@ -204,6 +207,7 @@ gtk_gesture_single_handle_event (GtkEventController *controller,
     case GDK_TOUCH_CANCEL:
     case GDK_GRAB_BROKEN:
     case GDK_TOUCHPAD_SWIPE:
+    case GDK_SCROLL:
       return GTK_EVENT_CONTROLLER_CLASS (gtk_gesture_single_parent_class)->handle_event (controller, event, x, y);
       break;
     default: