I'll just wrap things up quickly for the zero-sum politics series.
1) Trump can't lose gracefully because everything is zero-sum to him. Winning is all that matters.
2) He will accuse HRC of rigging the election should he lose.
3) He is probably too lazy to stir up any real trouble.
4) If he tries, the first limiting factor is how much his supporters believe in the basic constitutional framework of the US rather than just what they think of current leaders and policies.
5) Any real post-election protest movement based on a rigged-election claim would require organizing leaders at the ground level, who haven't emerged, and are unlikely to emerge.
6) The GOP establishment has incentives to stop him, but must begin before the election by not getting sucked into the rhetoric of "unskewed polls."
Basic conclusion: Trump will whine a lot, but he always does that. There will probably be a normal transition. But, there is a possibility of trouble. Trump has incited violence before, after all. Most likely, everything will be fine. I don't like adding those modifiers, though.
Albert Hirschman, Exit, Voice and Loyalty
Dennis Chong, Collective Action and the Civil Rights Movement
David Easton, A Systems Analysis of Political Life