Multiple calling of getGesturesStream causes memory leak and duplicate gestures to happen.

[NikitaSkoblov]

How to Reproduce:

1. Create a simple plot
2. Add some gestures source by calling mentioned method
3. Now reassign its gesturesoure by calling the same method multiple times

Expected: navigation works as usual
Observed: zoom "skyrockets" and is overaccelerated

the reason for this is that each new call of getgesturesstream adds its inner event call and subscribes to all previous calls of custom event "xbrowserwheel" so each call of those function adds duplicate gestures for each single mouse wheel event.