Environment variables should be checked for zero length,
instead of non-zero. This prevented the script to run
correctly in a jhbuild shell for example.
https://bugzilla.gnome.org/show_bug.cgi?id=693031
A new tool, 'gnome-shell-extension-prefs' can load a new entry point from
extensions, 'prefs.js', which has an entry point to return a GTK+ widget.
This allows extensions to have their own preferences dialog, without each
extension needing to ship its own Python script and .desktop file.
https://bugzilla.gnome.org/show_bug.cgi?id=668429