Review nimble-components for leaked subscriptions

[m-akinc]

🧹 Tech Debt

Recently, while investigating a memory leak, we discovered a couple places where we were not unsubscribing elements that had earlier subscribed for notifications. This contributed to the memory leak by holding references to those still-subscribed, dead elements.

We need to search the repo for calls to subscribe and make sure that there are matching calls to unsubscribe in the correct places.

One example that needs fixing is in table-column enum-base, as removeMappingObservers() is only called when replacing mapping observers. It should also be called when disconnecting the element.