gtk3: modify deprecated constants

gtk3/sample/misc/bindings.rb
gtk3: modify deprecated class names

gtk3/sample/misc/bindings.rb
gtk3: modify method calls with deprecated argument style

gtk3/sample/misc/binding.rb
gtk3: modify deprecated method calls

gtk3/sample/misc/bindings.rb
* Remove a Gtk::BindingSet#add_path method call.
* Replace Gtk::BindingSet#entry_clear method calls with Gtk::BindingSet#entry_remove.
Merge pull request #246 from kitachro/bindingsrb

gtk3: make one sample script gtk3 ready

Patch by kitachro. Thanks!!!