Fix for #1758876: client insensitive after discard

I suppose the problem was in the robber patch, although I don't really see how. Anyway, I've found and fixed it in the attached patch; after rolling a 7, the client goes in "waiting for network" state, and in case a discard was started, it didn't exit from that. I solved this by leaving the state when pushing mode_discard to the stack. This feels a bit wrong, because it is not near the place where the waiting for network state is entered, but it seems the best option.

While testing, I also found that in some cases the client went insensitive after a reconnect. In particular, when you have rolled a 7, and must discard. The problem was that after discard, the client is sent back to previous_state, which was frontend_gui_offline. I've fixed this in the patch as well.