An sevent sid_keysig sets the key signature for the track.  its data field
is a UBYTE number encoding a major key:

        data    key     music notation  data    key     music notation

         0  C maj
         1  G        #               8         F        b
         2  D        ##              9         Bb       bb
         3  A        ###            10      Eb       bbb
         4  E        ####           11      Ab       bbbb
         5  B        #####          12      Db       bbbbb
         6  F#       ######         13      Gb       bbbbbb
         7  C#       #######        14      Cb       bbbbbbb

A SID_KeySig sevent changes the key for the following notes in that track.
C major is the default key in every track before the first SID_KeySig
SEvent.