Update the Spawner Minecart

At present, Spawner Minecarts are missing a core functionality that regular Spawners have: the ability to change the mob they spawn with Spawn Eggs. Adding this feature to Spawner Minecarts would make it much easier to work with the entity, especially for content creators.

Spawner Minecarts are also missing a respective item in the Creative inventory. It should have one that can be used to place the entity, just like the Command Block Minecart and all of the other minecarts.