Start local server using client GUI
In order to facilitate #9 and MP gameplay improvements, it should be possible to start a local server also via the client GUI.
In essence, this means the client can also act as a minimal Shell application.
libshell needs to be enhanced to avoid any possible UI/feature redundancy between the Shell apps and the client's server manipulation UI.
This feature only covers ad-hoc servers: the server will be stopped when the hosting player quits.