DUBLIN (Reuters) - To restore faith in the 737 MAX, Boeing needs to prove its flagship jet is not just airworthy but also a safe investment.At a gathering in Dublin this week of the titans of the multibillion-dollar aircraft leasing industry, which finances half the world's fleet, cracks were appearing in that effort. Boeing said on Tuesday its troubled workhorse - grounded last March after two crashes in which 346 people died - should receive approval by mid-year from U.S regulators, paving..