Cleanup ClientLauncher

[SmallJoker]

ClientLauncher handles the client-relevant CLI arguments and main menu information to start a client.
This PR now removes duplicated variables and unifies the startup data into a new (inherited) struct. It's still a mess, but I hope this goes into the right direction.

Features:

• Easier to join singleplayer worlds (see examples below)

Fixes #8273 as a side-effect.

To do

This PR is Ready for Review.

How to test

# Singleplayer
minetest --go --worldname WORLDNAME

# Singleplayer: Ignored name input
minetest --go --worldname WORLDNAME --name foobar

# Remote server
minetest --go --address daconcepts.com --port 30001 --name Krock --password hunter2

# Remote server: Port and login from minetest.conf
minetest --go --address daconcepts.com

# Local server
minetest --server --worldname WORLDNAME --port 30001

# Local server: Port from minetest.conf
minetest --server --worldname WORLDNAME

Plus all main menu functions:

• Local server
• Singleplayer
• Join remote server

[sfan5]

In file included from .../minetest/src/client/clientlauncher.cpp:33:
.../minetest/src/client/clientlauncher.h:55:17: warning: private field 'font' is not used [-Wunused-private-field]
        gui::IGUIFont *font = nullptr;
                       ^
.../minetest/src/client/game.cpp:879:7: warning: private field 'random_input' is not used [-Wunused-private-field]
        bool random_input;
             ^

[sfan5]

Works.

[shacknetisp]

Is there a way with this cleanup to run a local game with a specific player name?

Previous I could use something like: minetest --go --worldname someworld --name beha --address "" to use the player beha instead of singleplayer

Now I get the error: ERROR[Main]: No world selected and no address provided. Nothing to do.