Update main menu

This commit changes a few things to the main menu:
- Removes unused files (the code is mostly within `init.lua`)
- Gets rid of the horrible dirt fallback background and uses the clouds instead
- Moves some stuff around for easier editing
- Renames some functions
- Changes the name of the `Client` tab to the `Multiplayer` tab (easier to understand)
- Changes the name of the `Server` tab to the `Create Server` tab (easier to understand, people might think it is for joining a server)
- Adds enable_node_highlighting to the settings tab