Client/cross-platform: Allow for cross-platform windowing

[twostars]

There's a lot of nuance to how it works directly with the Windows API. I don't necessarily want to just go back to SDL, as it creates so many support/compatibility issues with official behaviour.

As the client is both very particular and very simplistic, it might be feasible to roll our own platform-specific windowing. I would much prefer to at least continue to handle windowing with the WinAPI directly in Windows if possible.