Introduce a common settings.mk file, which is included in extensions
that need GSettings, to ensure that conventions are respected, and
necessary fixes can be applied from one place.
Now, typing "make zip-file" will create a standard zip-file for each
extension, with everything necessary, including translations and GSettings schemas.
These files can then be installed with the tweak-tool or uploaded
at extensions.gnome.org
Based on an earlier patch by Jasper St. Pierre.
