gdk3: add constants in Gdk::Selection

GitHub: fix #254

Related commits are the following:

  * gdk3: try to add constants in Gdk::Selection (not working)
    * commit 2c1602aa96779617db66258dd823cadbd5e41607

  * gi: support non GObject struct as return value
    * commit b9e1643b8b1ee4a3a2b2501975e5803552cd4e48