gtk3: don't use "Gdk::Window#size" as obsoleted
gtk3: don't use "Gdk::Window#size" as obsoleted

Use "Gdk::Window#width" and "Gdk::Window#height" since GDK 2.24.

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