change API string to avoid gnome-shell conflict

This pull request changes the Muffin API to avoid a conflict with the API for gnome shell.  It should be applied with the corresponding patch in cinnamon.

Although the API is private, having the Meta API set to 3.0 causes packaging issues with RPM based distros that automatically scan for gobject dependencies.  This can be worked around by a define in the RPM to avoid auto-dependencies, but this fix does things the right way by setting up a new API string that is unique to muffin.  Applying this patch removes the need for further internal name space changes.