iconhelper: Invalidate on clear()

When clearing the icon helper, we need to invalidate it. Otherwise the
previous icon keeps lingering along.
This is not relevant inside gtkiconhelper.c where other code causes the
invalidation, but happens when external code calls
_gtk_icon_helper_clear().

https://bugzilla.gnome.org/show_bug.cgi?id=765066

gtk/gtkiconhelper.c

@@ -103,6 +103,8 @@ _gtk_icon_helper_clear (GtkIconHelper *self)
   self->priv->def = gtk_image_definition_new_empty ();
 
   self->priv->icon_size = GTK_ICON_SIZE_INVALID;
+
+  gtk_icon_helper_invalidate (self);
 }
 
 static void