Possibly add a requirement for elementsToRerender in canvas strategy output

[Rheeseyb]

When adding new canvas strategies it is very easy to forget to use the setElementsToRerenderCommand when required (which is most of the time). The CustomStrategyState already has a field elementsToRerender. It might make more sense to remove the command, and to bring elementsToRerender up into the StrategyApplicationResult to enforce a conscious decision around whether elements need to be re-rendered as a result of applying this strategy.