Implement joining over Steam

This PR implements joining over the Steam friend list based on #159

Check for availability of steam_api64.dll or steam_api.dll and enable Steam feature accordingly
Set Steam rich presence values when acting as server or client:
- status: The fact that the user is playing on a server
- connect: A connection token to enable the "Join Game" button in the friend list
- steam_player_group: The same token identifying the current server, this will group all players on the same server in the friend list
- steam_player_group_size: The number of players on the server (Will show "+X other players" in the friend list grouping)

For other friends it looks like this: grafik (The text at the bottom says "Playing with 1 other player")

Implement server tokens
- Server generates random token
- Token is sent to joining clients
- All members provide the token to Steam
- The token is registered on the API server
- Joining over Steam then works with the token instead of an IP address and port
Read command line once in main menu to allow joining over the friend list even if the game is not running
Listen for Steam rich presence join requests
Include certain copied files from Steamworks.NET to implement the callback listener
Make API server port configurable

To test this PR, change the API server port to 4250.

CitiesSkylinesMultiplayer / CSM