A new Block class

[pltxtra]

Maybe a better way to store a block is to store it as a double linked list of events.
Each event has a time-stamp, and possibly a duration (for note on/note off type events so they
can be stored as "one" event, and we eliminate the matching problem.). Then the
sequence can store a pointer to the "next event" and the "previous event" for speed up.

This way of storing a block would increas resolution since it does not restrict an event to
a line in the block, or even a "tick" if the time stamp is a double precision floating point value.
It also eliminates the need for multiple tracks in the block and thus it enables the "tracker"
to show parallell events for choords and such.

[pltxtra (pltxtra)]

This has been implemented.