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
Merge pull request #993 from Joern-R/pull4

Fix context menu in ride editor (QT5 problem only)