Changed the meaning of the Modifiers keys to be positional, added Super, and pinned down Meta vs. Alt. All but the Meta/Alt one were done after discussion in #go-nuts about how to handle differences in user expectation properly. This is the portable interface only; each platform implementation comes next.
Added (most of) the Super and Meta changes for GTK+. The Modifiers were already in the correct order.
Added proper Super handling on Windows. Meta isn't used here and the modifiers were already in the correct order.
Implemented the Super and order changes on Mac OS X. Meta is irrelevant here.