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