Added new ServicesProposal library

- needed for bnc#887688
- Yast Network will have to set it (sshd)
- Yast Services Manager will have to read it and export it within its own configuration
- The test passes
- The package builds
And I think it would be easier to have two separate lists (`Set`s) for enabled and disabled services. And you could simply use accessors instead of `enabled_services` and `disabled_services` methods:

```ruby
require "set"
...

attr_reader :enabled_services, :disabled_services

def initialize
  @enabled_services = Set.new
  @disabled_services = Set.new
end

def enable_service(service)
  check_service(service)
  @enabled_services << service
end
```