Event edit gui works more.
The graphical feedback for box select, line, delete, and
insert work. Buttons C (clear) and X (clear all) were
added.
shift left drag: draw a box around events.
ctrl left drag: choose were to insert event and what value
right click drag: choose range of events to delete
left drag: draw a line to edit event values
middle click drag: choose where to past current selection
C button: clear all events of the current type shown in
the event editor.
X button: clear all events of all type (except note on/off)
The actual actions associated with the graphics do not
yet work.