Added Grid::_event_select() to find first selected event.
Changed Grid::set_duration() to Grid::set_sel_duration() to act on single selected note.
Now using Grid::set_duration() in Canvas::handle() which fixes bug which occurred when ALT click dragging the duration of a note.
Changed Grid::adj_velocity() to Grid::set_sel_velocity() which sets velocity of the current selected note.
Canvas::handle() now using more optimized set_sel_velocity().