Fix context menu in ride editor (QT5 problem only)

... same problem as in 'Ride Navigator'  (only QT 5.3.1) (fixed on June 20th - Commit see below)
... tested for 4.8.6 and 5.3.1 on Windows 

https://github.com/GoldenCheetah/GoldenCheetah/commit/88e1dc5745462a752c78c3d8c9ff9cc5d0a393b6