Moved platform-specific WWKeyboardClass code into their own classes and files.

[isojalka]

No new functionality is introduced with this pull request. It simply abstracts the WWKeyboardClass interface and moves existing code into platform-specific files, to avoid hard-to-maintain #ifdef blocks in the main file.

Please note that this commit is more than a year old at this point. I did test these changes back then for all platforms, and I briefly retested them just now with SDL2 and SDL1 on Linux. I'd appreciate it if someone who uses Vanilla Conquer on Windows could test this and confirm that it also still works there.