Fix memory leak using one-shot systems

[janhohenheim]

Also implements #234

Tried adding a closure directly to fn button, but that is rather cumbersome, as ChildBuilder does not have a register_one_shot_system method directly, which means we would have to re-implement it:

let entity = self.spawn_empty().id();
self.push(RegisterSystem::new(system, entity));
SystemId::from_entity(entity)

which is very very meh

[janhohenheim]

This here is the line that fixes the leak, as otherwise we never clean up the entities holding our one-shot systems but continue to register new ones when the menu is reconstructed.

[benfrankel]

Wording nit