[#127] Improve 'dist' target to create tarball

Created by: elbenfreund

Our dist target now also create a tarball of the extension that can be extracted right into ~/.local/share/gnome-shell/extensions/ if we want to install the extension manually and not via extensions.gnome.org. This avoids people clone the repository itself as done previously, which would not work anymore anyways as the new structure does not match what gnome shell extensions should look like.

Those new tarballs should be discributed as primary releases on github in order to complement extensions.gnome.org.

Closes: #127 (closed)