Patch-level plug-ins

[mokou]

Allow patches to contain DLLs which are loaded into the game process. This will drastically enhance what patches can do: For example, the British English patch could be generated automatically by adding a DLL that applies a set of text replacement rules to every line of text delivered from the engine.
#10 is a prerequisite for this. We should not add a feature that executes arbitrary binaries downloaded from arbitrary servers without a way to prove their authenticity.