Core: Avoid (Some) Global System Accessor

[mitaclaw]

Part 1 of an effort to remove global state from one of the biggest offenders.

[AdmiralCurtiss]

While I'm not entirely sure what your end goal looks like here (and frankly, even I'm not entirely sure how the Host <-> System interaction will end up looking by the end), I suppose this is fine for reducing the global accessor a bit.

[mitaclaw]

The end goal will be not a single Core::System::GetInstance() call in Core.cpp, and I am confident I can get it done. Other than Core::DisplayMessage, I have the whole plan mapped out to completion. It will mean Core::System::GetInstance() calls appearing in a few new files, but the idea is to gradually expose what components currently assume these functions are free to call (thus don't properly have access to System), and work forward from there.