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.
gtk3: GTK_TYPE_LEVEL_BAR is a GObject

So, use GTK_LEVEL_BAR(RVAL2GOBJ(o)) instead.
gtk3: has_offset_value is inconvenient function.

 Instead of implement offset_value to get offset_value.
gtk3: use `G_DEF_CLASS` for `enum GTK_TYPE_LEVEL_BAR_MODE`
gtk3: DBL2RVAL is not defined. use DBL2NUM instead.
gtk3: create a generic constructor

`Gtk::LevelBar.new` should work with both `Gtk::LevelBar.new()` and `Gtk::LevelBar.new(min_value, max_value)`.
Merge pull request #219 from cosmo0920/implement-gtk3-level-bar

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