gtk3 sample: make a sample script gtk3 ready

gtk3/sample/misc/dialog2.rb:

* modify method calls with a deprecated argument style
* modify deprecated constant names
* modify deprecated method names
* remove Gtk::Dialog::NO_SEPARATOR constant because it is obsolete
gtk3 sample: make a sample script gtk3 ready

gtk3/sample/misc/dialog.rb:

* modify a method call with a deprecated argument style
* modify a deprecated method name
* modify deprecated constant names