Fix context menu in ride editor (QT5 only)

... same problem as in 'Ride Navigator'  (only QT 5.3.1) (fixed on June
20th)
... tested both 4.8.6 and 5.3.1 now
...
https://github.com/GoldenCheetah/GoldenCheetah/commit/88e1dc5745462a752c78c3d8c9ff9cc5d0a393b6