ensures each combo in a sequence is in a predictable order and formats
key combos to be '+'-delimited

modifies the sequence in-place

@param {Array} sequence
@returns void
function _normalizeSequence(sequence) {
        var i;

        for (i = 0; i < sequence.length; ++i) {
            sequence[i].sort(function(x, y) {
                // modifier keys always come first, in alphabetical order
                if (x.length > 1 && y.length === 1) {
                    return -1;
                } else if (x.length === 1 && y.length > 1) {
                    return 1;
                }

                // character keys come next (list should contain no duplicates,
                // so no need for equality check)
                return x > y ? 1 : -1;
            });

            sequence[i] = sequence[i].join('+');
        }
    }