'dist' make target should produce tarball alongside 'zip'

Created by: elbenfreund

As we want to establish a clear distinction between source code and compiled extension (that includes i18n and schemata) we want to provide a way for users to 'install' the extension manually (that is: not via extensions.gnome.org). The solution for that was to just clone and symlink the repository. With our new approach this is no longer sensible (that is a good thing!). Instead, we should provide a ready-to-use tarball of the compiled extension ready to be extracted at the users leisure with every release.