Change decoder to not print 2 chars on overidden keycodes

[aurelivia]

As documented in #17, certain keycodes which are otherwise handled by the specialChar function are still producing an additional character. This changes that behaviour such that if a key is otherwise handled by specialChar it will not also produce an event for it's ASCII code, as it seems strange to do that when there is already an enum representing the key.

There is an additional fix in the PR for the windows Platform.hsc file as it seems to have been missed in a previous change and no longer compiles.

Closes #17