Implement online multiplayer

Things to note:

• Everything is "online multiplayer" now. This means that playing locally will run a game server locally as well (It currently opens a port on localhost as there's no way to have a "dummy" network connection) This has the benefit of a unified API. If it works locally, it should also work when deployed on a server.
• Everything is now divided into client, server and common. This brings some structure and we can produce smaller server exports by excluding the client code.
• Reworked a lot of internal structures

Breakages:

• Savegames currently don't work. They will likely be added back for local multiplayer in the near future. Adding savegame support to online multiplayer is more complicated.
• Leaving/Ending a game is not yet possible. This will need some UI rework (which will be the next step)

Documentation will follow shortly

Resolves #1 (closed)