Change delegates to weak references

[robhor]

SpotlightViewController contained a strong reference to a SpotlightTransitionController, which in turn had a strong reference to the SpotlightViewController through the delegate variable.
This caused a reference cycle that prevented SpotlightViewControllers from being deallocated, leaking memory.

By changing the delegate variable in SpotlightTransitionControllerDelegate to a weak variable, this retain cycle is prevented. I changed the delegate in SpotlightViewControllerDelegate to a weak variable as well, as this is common practice for delegates.

[xai3]

@robhor I was careless... thx!