Implement onCellContentChanged Engine Hander

Adds onCellContentChanged, an engine handler that fires whenever a cell gains or loses content. The handler receives (cell, changeType, objectType), so mods can specify the type of change they are looking for (added, removed, enabled, disabled, moved-in, moved-out), as well as the type of objects they care about. Leaving either the changeType or objectType field nil subscribes you to all values of either.

This mainly exists for removing the need for polling timers (or worse, onUpdate) for mods that need to keep track of a cell's content letting them be more accurate and more performant. I'm mainly using this for cache invalidation in my own mod which is its main use, but I know it should also be useful for mods like Mercy that need to keep track of Actors.