gi: maybe suppress the following warnings on armv7hl

    rb-gi-function-info.c: In function 'rb_gi_function_info_invoke_raw_call':
    rb-gi-function-info.c:579:32: warning: cast increases required alignment of target type [-Wcast-align]
                                    (GIArgument *)(data->in_args->data),
                                    ^
    rb-gi-function-info.c:581:32: warning: cast increases required alignment of target type [-Wcast-align]
                                    (GIArgument *)(data->out_args->data),
                                    ^

GitHub: #216

Reported by mtaska. Thanks!!!
gi test: loose test strictness to work on i686, x86_64 and armv7hl

GitHub: really fix #216

Reported by mtasaka. Thanks!!!
gtk3: remove unused functions

`activate_link_func` and `link_func` are no longer used.
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
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 ","
gtk3 test: remove needless assignment
gtk3: add Gtk::LevelBar.for_interval
gtk3 test: add Gtk::LevelBar tests
gtk3 test: test_gtk_levelbar.rb -> test_gtk_level_bar.rb
gtk3 test: don't use default value in test

Because 0.0 is the default value: https://developer.gnome.org/gtk3/stable/GtkLevelBar.html#gtk-level-bar-get-offset-valueg
gtk3 test: use more Ruby-ish API in test.

G_DEF_CLASS defines both .set_XXX and .XXX= accesssors.