gdk3: add missing deprecations
gdk3: add missing deprecations

  * Gdk::EventType related constants such as Gdk::Event::BUTTON2_PRESS
  * Gdk::EventMask related constants such as Gdk::Event::EXPOSURE_MASK

GitHub: fix #291

Reported by Michel Boaventura. Thanks!!!

kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2