gtk3: remove warning Gtk::Accessible

In GtkAccessible, `widget` is property. see: https://developer.gnome.org/gtk3/3.4/GtkAccessible.html
gtk3: remove warning Gtk::Entry

In GtkEntry, `completion` is property. see: https://developer.gnome.org/gtk3/3.4/GtkEntry.html
gtk3 test: add license headers
gtk3: remove warning Gtk::TreeSelection

In GtkTreeSelection, `mode` is property. see: https://developer.gnome.org/gtk3/3.4/GtkTreeSelection.html
gtk3: remove trailing spaces