Add Nautilus actions extension

Include a context menu item for opening directories, and one for opening
supported files, in a Nautilus extension.

https://bugzilla.gnome.org/show_bug.cgi?id=731493
Minor improvements to the Nautilus extension

Document the requirements for the extension in the README. Add a
COPYING.GPL3, as including the license is a requirement. Update
.gitignore for new files.
Use type definition macro for Nautilus extension

Use G_DEFINE_DYNAMIC_TYPE_EXTENDED for the Nautilus extension, and move
all the code into a single file as a consequence.
Initialize gettext in Nautilus extension startup
Add generated help translations to .gitignore