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
Update British English translation
Updated Greek translation
Update NEWS for future 2.3.1 release
Move main window to EtApplicationWindow object
Pass the main window to actions as user data
Move log area to EtLogArea object
Move playlist window to EtPlaylistDialog object
Move load filenames dialog to EtLoadFilesDialog
Move file search dialog to EtSearchDialog object
Move preferences dialog to EtPreferencesDialog
Move CDDB dialog to EtCDDBDialog object
Move scan dialog to EtScanDialog object
Replace strncat() with g_string_append()
Move select and unselect actions to main window
Move file selection actions to EtApplicationWindow