Create an action to notify about new Shell theme changes

[tfuxu]

An action that will periodically check for changes to theme in currently supported versions of GNOME Shell would definitely be useful, as sometimes a Shell theme may be updated in some hot-fix for the major version of Shell, which may require from us to manually intervene to update template files to match updated theme source files. This action could, if it detects an change, create an issue in this repo notifying us about new changes.