Installation clients

Ah, you are right. My concern was, "If I am adding a new module to the installation proposal, do I need to implement it or not?" In that case, can we leave the method out entirely? The only usage left seems to be
```ruby
        WFM.call("clone_proposal", ["Write"])
```

in yast/yast-installation/src/clients/clone_finish.rb:53, which can be handled specially without this library.