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!!!
gtk3: add a missing space after ","
gtk3 test: remove needless assignment
gtk3 test: use accessor for testing Gtk::MenuButton

We need to make Ruby/GIO GObject Introspection ready to test
Gtk::MenuButton#menu_model because it uses GMenuModel that is defined
in GIO.
gtk3: implement Gtk::LevelBar

Gtk::LevelBar since Gtk+3 3.6
gtk3 test: add gtk version check

Because GtkLevelBar is since GTK 3.6.
gtk3: add GTK_CHECK_VERSION(3,6,0)

Because GtkLevelBar is since GTK 3.6.
gtk3: add a missing space after ","