The look on First Lady Melania Trump’s face after a gunman tried to storm the White House Correspondents’ dinner Saturday night said it all.  Eyes wide, mouth agape, her features frozen in horror as the room in front of her erupted in chaos, tables flipping, crockery smashing, hundreds of journalists and politicians hitting the deck as Secret Service agents barrelled toward the stage, guns drawn.  By contrast, the expression on the president’s face was calm, slightly quizzical, his jaw tight, as he surveyed the ballroom and then glanced toward the agents dashing to block him with their bodies from potential other assailants before bundling him off stage.  Perhaps, after surviving his third confirmed assassination attempt in less than two years, Donald Trump is used to people trying to kill him — or perhaps he has just trained himself to have a poker face in any situation.  Either way, he was built for such moments of crisis.…