gtk3: implement Gtk::MenuButton

Gtk::MenuButton since Gtk+3 3.6
gtk3: remove trailing spaces
gtk3 test: add only_gtk_version

Because Gtk::MenuButton is since gtk 3.6.
gtk3: rename menubutton

menubutton ->
menu_button

rbgtk-menubutton.c ->
rbgtk-menu-button.c
gtk3: fix copyright informantion

Because this file is created by RUby-GNOME2 Project Team at 2014.
gtk3 test: remove needless assignment
gtk3 test: rename menubutton -> menu_button
Merge pull request #218 from cosmo0920/implement-gtk3-menu-button

Implement gtk 3.6 feature: Gtk::MenuButton

Patch by cosmo. Thanks!!!