Alternative license/GPL exception

[olofson]

In addition to replacing all remaining code that I (David Olofson) don't have the sole copyright for, the license should be changed as needed to allow the distribution of binaries linked with proprietary code, such as the Steamworks SDK.

While this does not strictly have to go into the public repository, as I could technically do whatever I want with my own code, it should be in here, to avoid having to discuss terms individually for any external code contributions.