Remove obsolete comment.

This was fixed a while ago already. So lets get it out to avoid confusion.
Basic implementation for adjustable strums.

There is still room for improvement (e.g. circle position is not updated on
strum adjustment). However, basic adjustment of an existing strum is now
working.
Show the control point during strum adjustment.
Make sure that upper and lower boundaries are respected as well.

THe left and right boundaries (i.e. left and right axes of an segment)
were respected. That is, a strum can only be placed between two axes and
not start in one segment and end in the next. This wasn't true yet for
upper and lower boundaries. This is fixed now, and a strum can not end
above or below the axes any longer.
There is no such canvas anymore.

This canvas was removed earlier on in favor of a rect to capture the mouse
events.
Give a proper identifier to the rect.
Update the test suite to reflect the changes in strum code.