The morning after Donald J. Trump canceled his rally in Chicago amid unruly protests, Hillary Clinton delivered her most extensive remarks to date denouncing the leading Republican candidate and the clashes that have erupted at his campaigns, accusing Mr. Trump of committing “political arson.” “The ugly, divisive rhetoric we are hearing from Donald Trump and the encouragement of violence and aggression is wrong, and it’s dangerous,” Mrs. Clinton told supporters Saturday morning at the O’Fallon Park recreation complex here. “If you play with matches, you’re going to start a fire you can’t control. That’s not leadership. That’s political arson.”