Migrate GD.Print and Console.Write to Serilog

[WildWeazel]

Replace all (? I can't really think of a reason not to, other than to print an error from the logger) GD.Print and Console.Write* statements with logs as demonstrated in MainMenu.cs. Use appropriate log levels and try to be consistent with what we log and how it's phrased. Also remove the redundant "Hello/Goodbye logger" prints in LogManager now that logging is working. Note that since LogManager is now a Node, calls to ForContext should be made within Node callbacks even though it is static, to avoid calling before LogManager is ready.

Requires #267