The world’s biggest celebration of the performing arts begins on Friday in Edinburgh, with its most international line-up yet grappling with issues every bit as weighty as those that set the tone for the city’s first festival in 1947.Then, the scars of World War II were still healing. Now the globe faces grave new threats in the form of climate change and a divisive wave of political populism. Scottish comic and fringe performer Mark Nelson is tackling Britain’s experience of polarized views in “Brexit Wounds”, his take on the island’s chaotic departure from the European Union.As a subject for comedy he considers Boris Johnson too good to be true, labelling Britain’s Prime Minister – along with US President Donald Trump – as “almost beyond satire.” “They make so many gaffes themselves, anything they do is almost funnier than anything you can come up with,” he said.Nelson’s solution so far has been to let his daughter Isla deliver the killer lines in “News at Three,” a reference to the age she made her debut. It’s an online sensation, scoring more than 140 million views to date. Now six, Isla has “retired” to go to school. Of Johnson, she said: “He couldn’t do one of my jigsaws”. While the festival’s aim to boost morale through culture has been a constant, its structure has radically changed over the decades. In 1947 only eight theatre groups turned up uninvited, beginning the original fringe.