gio2 test: add Gio::MemoryInputStream test
gtk3 test: use constant for stock ID
gio2 test: add MemortOutputStream test
Add Gtk::Box#pack_end
gio2: fix loader method name

following regexp unexpectedly not matched `executable`.

```ruby
   when /\Acan_be_/
     method_name = "#{$1}?"
```

So, `Gio::ContentType#?` is unexpectedly generated.
`Gio::ContentType#executables?` is what we want.
add documentation
gdk3: support Gdk::Keyval module functions

GitHub: fix #265

Reported by Detlef Reichl. Thanks!!!
Merge pull request #266 from detlefreichl/packend

Add Gtk::Box#pack_end

Patch by Detlef Reichl. Thanks!!!
Merge pull request #267 from detlefreichl/gi-repo-doc

Add documentation to GObjectIntrospection::Repository

Patch by Detlef Reichl. Thanks!!!